diff mbox series

upstream source tracing: base process (patch 1/3)

Message ID 20230420062024.134035-1-alberto@pianon.eu
State New
Headers show
Series upstream source tracing: base process (patch 1/3) | expand

Commit Message

Alberto Pianon April 20, 2023, 6:20 a.m. UTC
From: Alberto Pianon <alberto@pianon.eu>

License compliance, SBoM generation and CVE checking require to be able
to trace each source file back to its corresponding upstream source. The
current implementation of bb.fetch2 makes it difficult, especially when
multiple sources are combined together.

This series of patches provides a solution to the issue by implementing
a process that unpacks each SRC_URI element into a temporary directory,
collects relevant provenance metadata on each source file, moves
everything to the recipe rootdir, and saves metadata in a JSON file.

The proposed solution is split into a series of patches, with the first
patch containing required modifications to fetchers' code and a
TraceUnpackBase class that implements the process, and the second patch
implementing the data collecting logic in a separate TraceUnpack
subclass. The final patch includes test data and test cases to
demonstrate the solution's efficacy.

Signed-off-by: Alberto Pianon <alberto@pianon.eu>
---
 bin/bitbake-selftest        |   1 +
 lib/bb/fetch2/__init__.py   |  57 +++++++-
 lib/bb/fetch2/crate.py      |   2 +
 lib/bb/fetch2/gitsm.py      |  24 +++-
 lib/bb/fetch2/hg.py         |   1 +
 lib/bb/fetch2/npm.py        |   1 +
 lib/bb/fetch2/npmsw.py      |  26 +++-
 lib/bb/fetch2/trace_base.py | 254 ++++++++++++++++++++++++++++++++++++
 lib/bb/tests/trace_base.py  | 227 ++++++++++++++++++++++++++++++++
 9 files changed, 584 insertions(+), 9 deletions(-)
 create mode 100644 lib/bb/fetch2/trace_base.py
 create mode 100644 lib/bb/tests/trace_base.py

Comments

Luca Ceresoli April 20, 2023, 8:56 a.m. UTC | #1
Hello Alberto,

thanks for your patches!

I'm not going into the details of the implementation but I have a few
comments on formal aspects.

First, I would recommend to remove the "(patch 1/3)" from the subject,
or it will be part of the final commit message when applying the patch
using 'git am'. This is something that won't make sense later on, when
one will look at the git history. 'git format-patch origin/master' will
generate patches with an appropriate header for you, and for patch 3
which is special you can do a manual handling but still keep the
standard format: "[PATCH 3/3] ...commit title..."

On Thu, 20 Apr 2023 08:20:24 +0200
"Alberto Pianon" <alberto@pianon.eu> wrote:

> From: Alberto Pianon <alberto@pianon.eu>
> 
> License compliance, SBoM generation and CVE checking require to be able
> to trace each source file back to its corresponding upstream source. The
> current implementation of bb.fetch2 makes it difficult, especially when
> multiple sources are combined together.
> 
> This series of patches provides a solution to the issue by implementing
> a process that unpacks each SRC_URI element into a temporary directory,
> collects relevant provenance metadata on each source file, moves
> everything to the recipe rootdir, and saves metadata in a JSON file.
> 
> The proposed solution is split into a series of patches, with the first
> patch containing required modifications to fetchers' code and a
> TraceUnpackBase class that implements the process, and the second patch
> implementing the data collecting logic in a separate TraceUnpack
> subclass. The final patch includes test data and test cases to
> demonstrate the solution's efficacy.

Your comment is identical for patches 1 and 2. Instead a commit message
should describe what the single patch/commit does. Being this a pretty
complex patch set you certainly can include a couple lines giving the
big picture, but the bulk of the text should really be about _why_ we
need the patch, and also about _what_ it does in case it is not obvious
from the diff.

> 
> Signed-off-by: Alberto Pianon <alberto@pianon.eu>
> ---
>  bin/bitbake-selftest        |   1 +
>  lib/bb/fetch2/__init__.py   |  57 +++++++-
>  lib/bb/fetch2/crate.py      |   2 +
>  lib/bb/fetch2/gitsm.py      |  24 +++-
>  lib/bb/fetch2/hg.py         |   1 +
>  lib/bb/fetch2/npm.py        |   1 +
>  lib/bb/fetch2/npmsw.py      |  26 +++-
>  lib/bb/fetch2/trace_base.py | 254 ++++++++++++++++++++++++++++++++++++
>  lib/bb/tests/trace_base.py  | 227 ++++++++++++++++++++++++++++++++
>  9 files changed, 584 insertions(+), 9 deletions(-)
>  create mode 100644 lib/bb/fetch2/trace_base.py
>  create mode 100644 lib/bb/tests/trace_base.py
> 
> diff --git a/bin/bitbake-selftest b/bin/bitbake-selftest
> index f25f23b1..6d60a5d2 100755
> --- a/bin/bitbake-selftest
> +++ b/bin/bitbake-selftest
> @@ -31,6 +31,7 @@ tests = ["bb.tests.codeparser",
>           "bb.tests.runqueue",
>           "bb.tests.siggen",
>           "bb.tests.utils",
> +         "bb.tests.trace_base",
>           "bb.tests.compression",
>           "hashserv.tests",
>           "layerindexlib.tests.layerindexobj",
> diff --git a/lib/bb/fetch2/__init__.py b/lib/bb/fetch2/__init__.py
> index 1a86d8fd..96a7c7b0 100644
> --- a/lib/bb/fetch2/__init__.py
> +++ b/lib/bb/fetch2/__init__.py
> @@ -27,6 +27,10 @@ import bb.persist_data, bb.utils
>  import bb.checksum
>  import bb.process
>  import bb.event
> +try:
> +    from .trace import TraceUnpack

The trace module is introduced in a later patch, which is not very good
practice as the git history would not be bisectable. Commits should
first introduce basic components, and later commits should start using
them, and in case there is a good reason to not do so, they should be
merged in a single commit.

Best regards,
Luca
Alberto Pianon April 20, 2023, 10:30 a.m. UTC | #2
Hello Luca,
thanks for your suggestions, I will definitely try to follow them, 
re-submitting the patches.
Best,
Alberto


Il 2023-04-20 10:56 Luca Ceresoli ha scritto:
> Hello Alberto,
> 
> thanks for your patches!
> 
> I'm not going into the details of the implementation but I have a few
> comments on formal aspects.
> 
> First, I would recommend to remove the "(patch 1/3)" from the subject,
> or it will be part of the final commit message when applying the patch
> using 'git am'. This is something that won't make sense later on, when
> one will look at the git history. 'git format-patch origin/master' will
> generate patches with an appropriate header for you, and for patch 3
> which is special you can do a manual handling but still keep the
> standard format: "[PATCH 3/3] ...commit title..."
> 
> On Thu, 20 Apr 2023 08:20:24 +0200
> "Alberto Pianon" <alberto@pianon.eu> wrote:
> 
>> From: Alberto Pianon <alberto@pianon.eu>
>> 
>> License compliance, SBoM generation and CVE checking require to be 
>> able
>> to trace each source file back to its corresponding upstream source. 
>> The
>> current implementation of bb.fetch2 makes it difficult, especially 
>> when
>> multiple sources are combined together.
>> 
>> This series of patches provides a solution to the issue by 
>> implementing
>> a process that unpacks each SRC_URI element into a temporary 
>> directory,
>> collects relevant provenance metadata on each source file, moves
>> everything to the recipe rootdir, and saves metadata in a JSON file.
>> 
>> The proposed solution is split into a series of patches, with the 
>> first
>> patch containing required modifications to fetchers' code and a
>> TraceUnpackBase class that implements the process, and the second 
>> patch
>> implementing the data collecting logic in a separate TraceUnpack
>> subclass. The final patch includes test data and test cases to
>> demonstrate the solution's efficacy.
> 
> Your comment is identical for patches 1 and 2. Instead a commit message
> should describe what the single patch/commit does. Being this a pretty
> complex patch set you certainly can include a couple lines giving the
> big picture, but the bulk of the text should really be about _why_ we
> need the patch, and also about _what_ it does in case it is not obvious
> from the diff.
> 
>> 
>> Signed-off-by: Alberto Pianon <alberto@pianon.eu>
>> ---
>>  bin/bitbake-selftest        |   1 +
>>  lib/bb/fetch2/__init__.py   |  57 +++++++-
>>  lib/bb/fetch2/crate.py      |   2 +
>>  lib/bb/fetch2/gitsm.py      |  24 +++-
>>  lib/bb/fetch2/hg.py         |   1 +
>>  lib/bb/fetch2/npm.py        |   1 +
>>  lib/bb/fetch2/npmsw.py      |  26 +++-
>>  lib/bb/fetch2/trace_base.py | 254 
>> ++++++++++++++++++++++++++++++++++++
>>  lib/bb/tests/trace_base.py  | 227 ++++++++++++++++++++++++++++++++
>>  9 files changed, 584 insertions(+), 9 deletions(-)
>>  create mode 100644 lib/bb/fetch2/trace_base.py
>>  create mode 100644 lib/bb/tests/trace_base.py
>> 
>> diff --git a/bin/bitbake-selftest b/bin/bitbake-selftest
>> index f25f23b1..6d60a5d2 100755
>> --- a/bin/bitbake-selftest
>> +++ b/bin/bitbake-selftest
>> @@ -31,6 +31,7 @@ tests = ["bb.tests.codeparser",
>>           "bb.tests.runqueue",
>>           "bb.tests.siggen",
>>           "bb.tests.utils",
>> +         "bb.tests.trace_base",
>>           "bb.tests.compression",
>>           "hashserv.tests",
>>           "layerindexlib.tests.layerindexobj",
>> diff --git a/lib/bb/fetch2/__init__.py b/lib/bb/fetch2/__init__.py
>> index 1a86d8fd..96a7c7b0 100644
>> --- a/lib/bb/fetch2/__init__.py
>> +++ b/lib/bb/fetch2/__init__.py
>> @@ -27,6 +27,10 @@ import bb.persist_data, bb.utils
>>  import bb.checksum
>>  import bb.process
>>  import bb.event
>> +try:
>> +    from .trace import TraceUnpack
> 
> The trace module is introduced in a later patch, which is not very good
> practice as the git history would not be bisectable. Commits should
> first introduce basic components, and later commits should start using
> them, and in case there is a good reason to not do so, they should be
> merged in a single commit.
> 
> Best regards,
> Luca
Luca Ceresoli April 21, 2023, 7:15 a.m. UTC | #3
Ciao Alberto,

On Thu, 20 Apr 2023 08:20:24 +0200
"Alberto Pianon" <alberto@pianon.eu> wrote:

> From: Alberto Pianon <alberto@pianon.eu>
> 
> License compliance, SBoM generation and CVE checking require to be able
> to trace each source file back to its corresponding upstream source. The
> current implementation of bb.fetch2 makes it difficult, especially when
> multiple sources are combined together.
> 
> This series of patches provides a solution to the issue by implementing
> a process that unpacks each SRC_URI element into a temporary directory,
> collects relevant provenance metadata on each source file, moves
> everything to the recipe rootdir, and saves metadata in a JSON file.
> 
> The proposed solution is split into a series of patches, with the first
> patch containing required modifications to fetchers' code and a
> TraceUnpackBase class that implements the process, and the second patch
> implementing the data collecting logic in a separate TraceUnpack
> subclass. The final patch includes test data and test cases to
> demonstrate the solution's efficacy.
> 
> Signed-off-by: Alberto Pianon <alberto@pianon.eu>

Testing this patchset on the autobuilders triggered failures on the
oe-selftest builds. Here are the logs:

https://autobuilder.yoctoproject.org/typhoon/#/builders/79/builds/5085/steps/14/logs/stdio
https://autobuilder.yoctoproject.org/typhoon/#/builders/80/builds/5034/steps/14/logs/stdio
https://autobuilder.yoctoproject.org/typhoon/#/builders/86/builds/5076/steps/14/logs/stdio
https://autobuilder.yoctoproject.org/typhoon/#/builders/87/builds/5114/steps/14/logs/stdio
https://autobuilder.yoctoproject.org/typhoon/#/builders/127/builds/1296/steps/15/logs/stdio

May you have a look at that?

Best regards,
Luca
diff mbox series

Patch

diff --git a/bin/bitbake-selftest b/bin/bitbake-selftest
index f25f23b1..6d60a5d2 100755
--- a/bin/bitbake-selftest
+++ b/bin/bitbake-selftest
@@ -31,6 +31,7 @@  tests = ["bb.tests.codeparser",
          "bb.tests.runqueue",
          "bb.tests.siggen",
          "bb.tests.utils",
+         "bb.tests.trace_base",
          "bb.tests.compression",
          "hashserv.tests",
          "layerindexlib.tests.layerindexobj",
diff --git a/lib/bb/fetch2/__init__.py b/lib/bb/fetch2/__init__.py
index 1a86d8fd..96a7c7b0 100644
--- a/lib/bb/fetch2/__init__.py
+++ b/lib/bb/fetch2/__init__.py
@@ -27,6 +27,10 @@  import bb.persist_data, bb.utils
 import bb.checksum
 import bb.process
 import bb.event
+try:
+    from .trace import TraceUnpack
+except (ModuleNotFoundError, ImportError):
+    from .trace_base import TraceUnpackBase as TraceUnpack
 
 __version__ = "2"
 _checksum_cache = bb.checksum.FileChecksumCache()
@@ -1279,6 +1283,7 @@  class FetchData(object):
         if not self.pswd and "pswd" in self.parm:
             self.pswd = self.parm["pswd"]
         self.setup = False
+        self.destdir = None
 
         def configure_checksum(checksum_id):
             if "name" in self.parm:
@@ -1554,6 +1559,8 @@  class FetchMethod(object):
             bb.utils.mkdirhier(unpackdir)
         else:
             unpackdir = rootdir
+        urldata.destdir = unpackdir
+        urldata.is_unpacked_archive = unpack and cmd
 
         if not unpack or not cmd:
             # If file == dest, then avoid any copies, as we already put the file into dest!
@@ -1569,6 +1576,7 @@  class FetchMethod(object):
                     if urlpath.find("/") != -1:
                         destdir = urlpath.rsplit("/", 1)[0] + '/'
                         bb.utils.mkdirhier("%s/%s" % (unpackdir, destdir))
+                        urldata.destdir = "%s/%s" % (unpackdir, destdir)
                 cmd = 'cp -fpPRH "%s" "%s"' % (file, destdir)
 
         if not cmd:
@@ -1852,26 +1860,69 @@  class Fetch(object):
             if not ret:
                 raise FetchError("URL %s doesn't work" % u, u)
 
-    def unpack(self, root, urls=None):
+    def unpack(self, root, urls=None, is_module=False, checkout_destdir=None):
         """
-        Unpack urls to root
+        Unpack urls to a tmp dir, trace, and then move everything to root
+
+        is_module needs to be set to true when this method is recursively called
+        by a fetcher's unpack method to unpack (sub)modules (gitsm, npmsw)
+
+        checkout_destdir needs to be passed when this method is recursively
+        called by gitsm fetcher
         """
 
         if not urls:
             urls = self.urls
+        if is_module:
+            destdir = root
+        else:
+            trace = TraceUnpack(root, self.d)
+            destdir = trace.tmpdir
 
         for u in urls:
             ud = self.ud[u]
+            # absolute subdir, destsuffix and subpath params wouldn't work when
+            # unpacking in the tmp dir, convert them to relative paths
+            realroot = os.path.realpath(root)
+            params = [ 'subdir', 'destsuffix', 'subpath' ]
+            for p in params:
+                if not ud.parm.get(p):
+                    continue
+                if os.path.isabs(ud.parm[p]):
+                    realpath = os.path.realpath(ud.parm[p])
+                    if realpath.startswith(realroot):
+                        ud.parm[p] = os.path.relpath(realpath, realroot)
             ud.setup_localpath(self.d)
+            ud.rootdir = root
+
+            if hasattr(ud, "checkout_destdir"):
+                ud.checkout_destdir = checkout_destdir
 
             if ud.lockfile:
                 lf = bb.utils.lockfile(ud.lockfile)
 
-            ud.method.unpack(ud, root, self.d)
+            ud.method.unpack(ud, destdir, self.d)
 
             if ud.lockfile:
                 bb.utils.unlockfile(lf)
 
+            if is_module:
+                continue
+
+            if hasattr(ud, "nocheckout") and ud.nocheckout:
+                logger.warning(
+                    "Can't trace sources for"
+                    " %s because repo has not been checked out" % u)
+            else:
+                trace.commit(u, ud)
+
+            trace.move2root()
+
+        if is_module:
+            return
+        trace.write_data()
+        trace.close()
+
     def clean(self, urls=None):
         """
         Clean files that the fetcher gets or places
diff --git a/lib/bb/fetch2/crate.py b/lib/bb/fetch2/crate.py
index a7021e5b..93666de0 100644
--- a/lib/bb/fetch2/crate.py
+++ b/lib/bb/fetch2/crate.py
@@ -101,8 +101,10 @@  class Crate(Wget):
         pn = d.getVar('BPN')
         if pn == ud.parm.get('name'):
             cmd = "tar -xz --no-same-owner -f %s" % thefile
+            ud.destdir = rootdir
         else:
             cargo_bitbake = self._cargo_bitbake_path(rootdir)
+            ud.destdir = cargo_bitbake
 
             cmd = "tar -xz --no-same-owner -f %s -C %s" % (thefile, cargo_bitbake)
 
diff --git a/lib/bb/fetch2/gitsm.py b/lib/bb/fetch2/gitsm.py
index f8e239bc..c1d291a1 100644
--- a/lib/bb/fetch2/gitsm.py
+++ b/lib/bb/fetch2/gitsm.py
@@ -34,6 +34,11 @@  class GitSM(Git):
         """
         return ud.type in ['gitsm']
 
+    def urldata_init(self, ud, d):
+        super(GitSM, self).urldata_init(ud, d)
+        ud.module_data = []
+        ud.checkout_destdir = None
+
     def process_submodules(self, ud, workdir, function, d):
         """
         Iterate over all of the submodules in this repository and execute
@@ -138,6 +143,15 @@  class GitSM(Git):
 
             function(ud, url, module, paths[module], workdir, ld)
 
+            if function.__name__ == "unpack_submodules":
+                destdir = os.path.join(ud.checkout_destdir, paths[module])
+                ud.module_data.append({
+                    "url": url,
+                    "destdir": destdir.rstrip("/"),
+                    "parent_destdir": ud.checkout_destdir.rstrip("/"),
+                    "revision": subrevision[module]
+                })
+
         return submodules != []
 
     def need_update(self, ud, d):
@@ -209,9 +223,13 @@  class GitSM(Git):
             else:
                 repo_conf = os.path.join(ud.destdir, '.git')
 
+            checkout_destdir = os.path.join(ud.checkout_destdir, modpath)
+
             try:
                 newfetch = Fetch([url], d, cache=False)
-                newfetch.unpack(root=os.path.dirname(os.path.join(repo_conf, 'modules', module)))
+                newfetch.unpack(root=os.path.dirname(os.path.join(repo_conf, 'modules', module)), is_module=True, checkout_destdir=checkout_destdir)
+                # add nested submodules' data
+                ud.module_data += newfetch.ud[url].module_data
             except Exception as e:
                 logger.error('gitsm: submodule unpack failed: %s %s' % (type(e).__name__, str(e)))
                 raise
@@ -233,6 +251,10 @@  class GitSM(Git):
 
         Git.unpack(self, ud, destdir, d)
 
+        if not ud.checkout_destdir:
+            # for main git repo, checkout destdir correspond with unpack destdir
+            ud.checkout_destdir = ud.destdir
+
         ret = self.process_submodules(ud, ud.destdir, unpack_submodules, d)
 
         if not ud.bareclone and ret:
diff --git a/lib/bb/fetch2/hg.py b/lib/bb/fetch2/hg.py
index 063e1300..0fd69db7 100644
--- a/lib/bb/fetch2/hg.py
+++ b/lib/bb/fetch2/hg.py
@@ -242,6 +242,7 @@  class Hg(FetchMethod):
         revflag = "-r %s" % ud.revision
         subdir = ud.parm.get("destsuffix", ud.module)
         codir = "%s/%s" % (destdir, subdir)
+        ud.destdir = codir
 
         scmdata = ud.parm.get("scmdata", "")
         if scmdata != "nokeep":
diff --git a/lib/bb/fetch2/npm.py b/lib/bb/fetch2/npm.py
index 8a179a33..34e1f276 100644
--- a/lib/bb/fetch2/npm.py
+++ b/lib/bb/fetch2/npm.py
@@ -294,6 +294,7 @@  class Npm(FetchMethod):
         destsuffix = ud.parm.get("destsuffix", "npm")
         destdir = os.path.join(rootdir, destsuffix)
         npm_unpack(ud.localpath, destdir, d)
+        ud.destdir = destdir
 
     def clean(self, ud, d):
         """Clean any existing full or partial download"""
diff --git a/lib/bb/fetch2/npmsw.py b/lib/bb/fetch2/npmsw.py
index 36fcbfba..79c369dc 100644
--- a/lib/bb/fetch2/npmsw.py
+++ b/lib/bb/fetch2/npmsw.py
@@ -66,6 +66,9 @@  class NpmShrinkWrap(FetchMethod):
     def urldata_init(self, ud, d):
         """Init npmsw specific variables within url data"""
 
+        # initialize module_data (for module source tracing)
+        ud.module_data = []
+
         # Get the 'shrinkwrap' parameter
         ud.shrinkwrap_file = re.sub(r"^npmsw://", "", ud.url.split(";")[0])
 
@@ -250,20 +253,33 @@  class NpmShrinkWrap(FetchMethod):
 
     def unpack(self, ud, rootdir, d):
         """Unpack the downloaded dependencies"""
-        destdir = d.getVar("S")
-        destsuffix = ud.parm.get("destsuffix")
-        if destsuffix:
-            destdir = os.path.join(rootdir, destsuffix)
+        # rootdir param is a temporary dir. The real rootdir, where sources are
+        # moved after being traced, is stored in ud.rootdir.
+        destsuffix = ud.parm.get("destsuffix") or os.path.relpath(d.getVar("S"), ud.rootdir)
+        destdir = os.path.join(rootdir, destsuffix)
+        ud.destdir = destdir
 
         bb.utils.mkdirhier(destdir)
         bb.utils.copyfile(ud.shrinkwrap_file,
                           os.path.join(destdir, "npm-shrinkwrap.json"))
 
+        for dep in ud.deps:
+            dep_destdir = os.path.join(destdir, dep["destsuffix"])
+            # to get parent destdir, we get rid of the last 2 path elements
+            # (node_modules/<module_name>)
+            dep_parent_destdir = "/".join(dep_destdir.split("/")[:-2])
+            ud.module_data.append({
+                "url": dep["url"] or dep["localpath"],
+                "destdir": dep_destdir.rstrip("/"),
+                "parent_destdir": dep_parent_destdir.rstrip("/"),
+                "revision": None
+            })
+
         auto = [dep["url"] for dep in ud.deps if not dep["localpath"]]
         manual = [dep for dep in ud.deps if dep["localpath"]]
 
         if auto:
-            ud.proxy.unpack(destdir, auto)
+            ud.proxy.unpack(destdir, auto, is_module=True)
 
         for dep in manual:
             depdestdir = os.path.join(destdir, dep["destsuffix"])
diff --git a/lib/bb/fetch2/trace_base.py b/lib/bb/fetch2/trace_base.py
new file mode 100644
index 00000000..d94c72b7
--- /dev/null
+++ b/lib/bb/fetch2/trace_base.py
@@ -0,0 +1,254 @@ 
+"""Module implementing a base process for upstream source tracing
+for bb.fetch2.Fetch.unpack()
+
+The process consists of:
+
+- creating a temporary directory where each SRC_URI element is unpacked
+
+- collecting relevant metadata (provenance) for each source file and for every
+  upstream source component, that can be used later on for Software Composition
+  Analysis, SBoM generation, etc.;
+
+- moving everything from the temporary directory to root, and iterate with the
+  next SRC_URI element;
+
+- saving metadata in a json file after all elements have been processed.
+
+It assumes that:
+
+- fetchers store unpack destination dir in urldata.destdir;
+- gitsm and npmsw fetchers store module metadata in urldata.module_data, as a
+  list of dict elements in the following format:
+    [
+        {
+            "url": "<module url>",
+            "destdir": "<module destination path>",
+            "parent_destdir": "<parent module destination path>"
+            "revision": "<git submodule revision (only for gitsm, else None)>"
+        }, ...
+    ]
+- urldata.is_unpacked_archive (boolean) is set to True or False for "file"
+  SRC_URI entries.
+"""
+
+# Copyright (C) 2023 Alberto Pianon <pianon@array.eu>
+#
+# SPDX-License-Identifier: GPL-2.0-only
+#
+
+import os
+import json
+import tempfile
+
+import bb.utils
+import bb.compress.zstd
+
+class TraceException(Exception):
+    pass
+
+def scandir(path):
+    with os.scandir(path) as scan:
+        return { e.name: e for e in scan }
+
+def is_real_dir(e):
+    return e.is_dir() and not e.is_symlink()
+
+def is_real_and_nonempty_dir(e):
+    return is_real_dir(e) and scandir(e.path)
+
+def is_file_or_symlink(e):
+    return e.is_file() or e.is_symlink()
+
+def is_git_dir(e):
+    path_scandir = scandir(e.path)
+    if ".git" in path_scandir:
+        try:
+            bb.process.run(
+                ["git", "rev-parse", "--is-inside-work-tree"], cwd=e.path)
+            return True
+        except bb.process.ExecutionError:
+            return False
+    return False
+
+def check_is_real_dir(path, name):
+    if not os.path.exists(path) or os.path.islink(path) or os.path.isfile(path):
+        raise TraceException(
+            "%s path %s is not a directory" % (name, path))
+
+def move_contents(src_dir, dst_dir):
+    """Move and merge contents from src_dir to dst_dir
+
+    Conflict resolution criteria are explained in bb.tests.trace_base
+
+    It's optimized for fast execution time by using os.scandir and os.rename, so
+    it requires that both src_dir and dst_dir reside in the same filesystem.
+    """
+
+    check_is_real_dir(src_dir, "Source")
+    check_is_real_dir(dst_dir, "Destination")
+
+    if os.lstat(src_dir).st_dev != os.lstat(dst_dir).st_dev:
+        raise TraceException(
+            "Source %s and destination %s must be in the same filesystem" %
+            (src_dir, dst_dir)
+        )
+
+    src_scandir = scandir(src_dir)
+    dst_scandir = scandir(dst_dir)
+
+    for src_name, src in src_scandir.items():
+        dst = dst_scandir.get(src_name)
+        if dst:
+            # handle conflicts
+            if is_real_dir(src) and is_real_and_nonempty_dir(dst):
+                if is_git_dir(src):
+                    bb.utils.prunedir(dst.path)
+                else:
+                    move_contents(src.path, dst.path)
+                    os.rmdir(src.path)
+                    continue
+            elif is_real_dir(src) and is_file_or_symlink(dst):
+                os.remove(dst.path)
+            elif is_file_or_symlink(src) and is_real_dir(dst):
+                try:
+                    os.rmdir(dst.path)
+                except OSError as e:
+                    if e.errno == 39:
+                        raise TraceException(
+                            "Error while moving %s contents to %s, cannot move"
+                            " %s to %s: source is a file or a symlink, while"
+                            " destination is a non-empty directory."
+                            % (src_dir, dst_dir, src.path, dst.path)
+                        )
+                    else:
+                        raise e
+        dst_path = dst.path if dst else os.path.join(dst_dir, src_name)
+        os.rename(src.path, dst_path)
+
+def findall_files_and_links(path, exclude=[], skip_git_submodules=False):
+    """recusively find all files and links in path, excluding dir and file names
+    in exclude, and excluding git dirs if skip_git_submodules is set to True.
+
+    Returns tuple of sorted lists of file and link paths (sorting is for
+    reproducibility in tests)
+    """
+    files = []
+    links = []
+    with os.scandir(path) as scan:
+        for e in scan:
+            if e.name in exclude:
+                continue
+            if e.is_symlink():
+                links.append(e.path)
+            elif e.is_file():
+                files.append(e.path)
+            elif e.is_dir():
+                if skip_git_submodules and is_git_dir(e):
+                    continue
+                _files, _links = findall_files_and_links(
+                        e.path, exclude, skip_git_submodules)
+                files += _files
+                links += _links
+    return sorted(files), sorted(links)
+
+class TraceUnpackBase:
+    """base class for implementing a process for upstream source tracing
+    See this module's help for more details on the process.
+
+    This base class implements the process but does not collect any data. It is
+    intended to be subclassed in a separate 'trace' module, implementing
+    _collect_data() and _process_data() methods.
+
+    Method call order:
+        - __init__(): initialize tmpdir and td (trace data)
+        - for each SRC_URI entry unpack:
+          - commit(): go through all files in tmpdir (and in each module subdir
+            in case of gitsm and npmsw fecthers) and commit collected metadata
+            to td
+          - move2root(): moves all files from tmpdir to root
+        - write_data()
+        - close(): delete tmpdir and cache
+    """
+
+    def __init__(self, root, d):
+        """initialize properties and create temporary directory in root
+
+        Temporary unpack dir is created in 'root' to ensure they are in the
+        same filesystem, so files can be quickly moved to 'root' after tracing
+        """
+
+        self.root = root
+        self.d = d
+        self.td = {}
+        if not os.path.exists(root):
+            bb.utils.mkdirhier(root)
+        self.tmpdir = tempfile.mkdtemp(dir=root)
+
+    def commit(self, u, ud):
+        """go through all files in tmpdir and commit collected metadata to td.
+        dive into module subdirs in case of gitsm and npmsw fecthers
+
+        Params are:
+        - u -> str: src uri of the upstream repo/package that is being processed
+        - ud -> bb.fetch2.FetchData: src uri fetch data object; ud.url and u do not correspond when git/npm modules are being processed, so we need both
+        """
+
+        exclude=['.git', '.hg', '.svn']
+
+        # exclude node_modules subdirs (will be separately parsed)
+        if ud.type in ['npm', 'npmsw']:
+            exclude.append('node_modules')
+        # exclude git submodules (will be separately parsed)
+        skip_git_submodules = (ud.type == 'gitsm')
+
+        files, links = findall_files_and_links(
+            ud.destdir, exclude, skip_git_submodules)
+        self._collect_data(u, ud, files, links, ud.destdir)
+
+        if ud.type in ['gitsm', 'npmsw'] and ud.module_data:
+            self._process_module_data(ud)
+            for md in ud.module_data:
+                files, links = findall_files_and_links(
+                   md["destdir"], exclude, skip_git_submodules)
+                self._collect_data(
+                    md["url"], ud, files, links, md["destdir"], md)
+
+    def _process_module_data(self, ud):
+        """add parent module data to each module data item, to map dependencies
+        """
+        revision = ud.revisions[ud.names[0]] if ud.type == 'gitsm' else None
+        indexed_md = { md["destdir"]: md for md in ud.module_data }
+        # add main git repo (gitsm) or npm-shrinkwrap.json (npmsw)
+        indexed_md.update({
+                ud.destdir.rstrip("/"): {"url": ud.url, "revision": revision}
+        })
+        for md in ud.module_data:
+            md["parent_md"] = indexed_md[md["parent_destdir"]]
+
+    def move2root(self):
+        """move all files from temporary directory to root"""
+        move_contents(self.tmpdir, self.root)
+
+    def write_data(self):
+        self._process_data()
+        if not self.d.getVar("PN"):
+            return
+        path = "%s/temp/%s-%s.unpack.trace.json.zst" % (
+            self.root, self.d.getVar("PN"), self.d.getVar("PV"))
+        with bb.compress.zstd.open(path, "wt", encoding="utf-8") as f:
+            json.dump(self.td, f)
+            f.flush()
+
+    def close(self):
+        os.rmdir(self.tmpdir)
+        del self.td
+
+    def _collect_data(self, u, ud, files, links, destdir, md=None):
+        """
+        collect provenance metadata on the committed files. Not implemented
+        """
+        pass
+
+    def _process_data(self):
+        """post-process self.td. Not implemented"""
+        pass
\ No newline at end of file
diff --git a/lib/bb/tests/trace_base.py b/lib/bb/tests/trace_base.py
new file mode 100644
index 00000000..d96fb2c7
--- /dev/null
+++ b/lib/bb/tests/trace_base.py
@@ -0,0 +1,227 @@ 
+
+# Copyright (C) 2023 Alberto Pianon <pianon@array.eu>
+#
+# SPDX-License-Identifier: GPL-2.0-only
+#
+
+import os
+import re
+import unittest
+import tempfile
+from pathlib import Path
+import subprocess
+
+import bb
+
+def create_src_dst(tmpdir):
+    src_dir = os.path.join(tmpdir, "src/")
+    dst_dir = os.path.join(tmpdir, "dst/")
+    os.makedirs(src_dir)
+    os.makedirs(dst_dir)
+    return Path(src_dir), Path(dst_dir)
+
+def make_dirname(path):
+    dirname = os.path.dirname(path)
+    if dirname:
+        os.makedirs(dirname, exist_ok=True)
+
+def create_file(path, content):
+    make_dirname(path)
+    with open(path, "w") as f:
+        f.write(content)
+
+def create_link(path, target):
+    make_dirname(path)
+    os.symlink(target, path)
+
+def get_tree(path):
+    curdir = os.getcwd()
+    os.chdir(path)
+    tree = []
+    for root, dirs, files in os.walk("."):
+        for f in dirs + files:
+            tree.append(re.sub(r"^\.\/", "", os.path.join(root, f)))
+    os.chdir(curdir)
+    return sorted(tree)
+
+def read_file(path):
+    with open(path) as f:
+        return f.read()
+
+class MoveContentsTest(unittest.TestCase):
+    """
+    Test the following conflict resolution criteria:
+
+    - if a file (or symlink) exists both in src_dir and in dst_dir, the
+      file/symlink in dst_dir will be overwritten;
+
+    - if a subdirectory exists both in src_dir and in dst_dir, their contents
+      will be merged, and in case of file/symlink conflicts, files/symlinks in
+      dst_dir will be overwritten - unless src_dir is a git repo; in  such a
+      case, dst_dir will be pruned and src_dir will be moved to dst_dir, for
+      consistency with bb.fetch2.git.Git.unpack method's behavior (which prunes
+      clone dir if already existing, before cloning)
+
+    - if the same relative path exists both in src_dir and in dst_dir, but the
+      path in src_dir is a directory and the path in dst_dir is a file/symlink,
+      the latter will be overwritten;
+
+    - if instead the path in src_dir is a file and the path in dst_dir is a
+      directory, the latter will be overwritten only if it is empty, otherwise
+      an exception will be raised.
+    """
+
+    def test_dir_merge_and_file_overwrite(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+            create_file(src_dir / "dir/subdir/file.txt", "new")
+            create_file(dst_dir / "dir/subdir/file.txt", "old")
+            create_file(dst_dir / "dir/subdir/file1.txt", "old")
+            bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+            expected_dst_tree = [
+                "dir",
+                "dir/subdir",
+                "dir/subdir/file.txt",
+                "dir/subdir/file1.txt"
+            ]
+            self.assertEqual(get_tree(src_dir), [])
+            self.assertEqual(get_tree(dst_dir), expected_dst_tree)
+            self.assertEqual(read_file(dst_dir / "dir/subdir/file.txt"), "new")
+            self.assertEqual(read_file(dst_dir / "dir/subdir/file1.txt"), "old")
+
+    def test_file_vs_symlink_conflicts(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+
+            create_file(src_dir / "dir/subdir/fileA.txt", "new")
+            create_file(src_dir / "dir/fileB.txt", "new")
+            create_link(src_dir / "file.txt", "dir/subdir/fileA.txt")
+
+            create_file(dst_dir / "dir/subdir/fileA.txt", "old")
+            create_link(dst_dir / "dir/fileB.txt", "subdir/fileA.txt")
+            create_file(dst_dir / "file.txt", "old")
+
+            bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+            self.assertEqual(get_tree(src_dir), [])
+            self.assertTrue(os.path.islink(dst_dir / "file.txt"))
+            self.assertEqual(
+                os.readlink(dst_dir / "file.txt"),
+                "dir/subdir/fileA.txt"
+            )
+            self.assertFalse(os.path.islink(dst_dir / "dir/fileB.txt"))
+            self.assertEqual(read_file(dst_dir / "dir/fileB.txt"), "new")
+
+    def test_dir_vs_file_conflict(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+            create_file(src_dir / "items/item0/content.txt", "hello")
+            create_file(dst_dir / "items/item0", "there")
+            bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+            self.assertEqual(get_tree(src_dir), [])
+            self.assertTrue(os.path.isdir(dst_dir / "items/item0"))
+            self.assertEqual(
+                read_file(dst_dir / "items/item0/content.txt"), "hello")
+
+    def test_dir_vs_symlink_conflict(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+            create_file(src_dir / "items/item0/content.txt", "hello")
+            create_file(dst_dir / "items/item1/content.txt", "there")
+            create_link(dst_dir / "items/item0", "item1")
+            bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+            self.assertEqual(get_tree(src_dir), [])
+            self.assertFalse(os.path.islink(dst_dir / "items/item0"))
+            self.assertEqual(
+                read_file(dst_dir / "items/item0/content.txt"), "hello")
+            self.assertEqual(
+                read_file(dst_dir / "items/item1/content.txt"), "there")
+
+    def test_symlink_vs_empty_dir_conflict(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+            create_file(src_dir / "items/item1/content.txt", "there")
+            create_link(src_dir / "items/item0", "item1")
+            os.makedirs(dst_dir / "items/item0")
+            bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+            self.assertEqual(get_tree(src_dir), [])
+            self.assertTrue(os.path.islink(dst_dir / "items/item0"))
+            self.assertEqual(read_file(dst_dir / "items/item0/content.txt"), "there")
+
+    def test_symlink_vs_nonempty_dir_conflict(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+            create_file(src_dir / "items/item1/content.txt", "there")
+            create_link(src_dir / "items/item0", "item1")
+            create_file(dst_dir / "items/item0/content.txt", "hello")
+            with self.assertRaises(bb.fetch2.trace_base.TraceException) as context:
+                bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+
+    def test_file_vs_empty_dir_conflict(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+            create_file(src_dir / "items/item0", "test")
+            os.makedirs(dst_dir / "items/item0")
+            bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+            self.assertEqual(get_tree(src_dir), [])
+            self.assertTrue(os.path.isfile(dst_dir/ "items/item0"))
+
+    def test_file_vs_nonempty_dir_conflict(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+            create_file(src_dir / "items/item0", "test")
+            create_file(dst_dir / "items/item0/content.txt", "test")
+            with self.assertRaises(bb.fetch2.trace_base.TraceException) as context:
+                bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+
+    def test_git_dir(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            src_dir, dst_dir = create_src_dst(tmpdir)
+            git_repo = src_dir / "src/my_git_repo"
+            create_file(git_repo / "foo.txt", "hello")
+            subprocess.check_output(["git", "init"], cwd=git_repo)
+            create_file(dst_dir / "src/my_git_repo/content.txt", "there")
+            bb.fetch2.trace_base.move_contents(src_dir, dst_dir)
+            self.assertFalse(
+                os.path.exists(dst_dir / "src/my_git_repo/content.txt"))
+                # git clone dir should be pruned if already existing
+            self.assertEqual(
+                read_file(dst_dir / "src/my_git_repo/foo.txt"), "hello")
+            self.assertTrue(os.path.isdir(dst_dir / "src/my_git_repo/.git"))
+
+
+class FindAllFilesAndLinksTest(unittest.TestCase):
+    """test if all files and links are correctly returned, and if specific
+    file/dir names and git subdirs are correctly excluded"""
+
+    def test_findall_files_and_links(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            tmpdir = Path(tmpdir)
+            files = {
+                str(tmpdir/"foo/example/example.txt"): "example",
+                str(tmpdir/"foo/foo.txt"): "foo",
+                str(tmpdir/"foo/foo2.txt"): "foo2",
+                str(tmpdir/"README"): "hello",
+            }
+            ignored = {
+                str(tmpdir/".git"): "fake",
+                str(tmpdir/"foo2/dummy"): "dummy"
+            }
+            allfiles = files.copy()
+            allfiles.update(ignored)
+            links = {
+                str(tmpdir/"example"): "foo/example", # link to dir
+                str(tmpdir/"example.txt"): "foo/example/example.txt", # link to file
+            }
+            for path, content in allfiles.items():
+                create_file(path, content)
+            for path, target in links.items():
+                create_link(path, target)
+            subprocess.check_output(["git", "init"], cwd=tmpdir/"foo2")
+            res_files, res_links = bb.fetch2.trace_base.findall_files_and_links(
+                    tmpdir, exclude=['.git'], skip_git_submodules=True)
+            self.assertEqual(res_files, sorted(list(files.keys())))
+            self.assertEqual(res_links, sorted(list(links.keys())))
+
+
+if __name__ == '__main__':
+    unittest.main()