fetch2/git: Avoid races over mirror tarball creation

There is a potential race over the mirror tarballs where a partial git repo
could be extracted causing fetcher failures if the tarball is being rewritten
whilst another build accesses it.

Create the mirror tarball atomically to avoid this.

[YOCTO #14441]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 19 insertions, 2 deletions

diff --git a/lib/bb/fetch2/git.py b/lib/bb/fetch2/git.py
index 488f4c741..3643a491d 100644
--- a/lib/bb/fetch2/git.py
+++ b/lib/bb/fetch2/git.py
@@ -68,6 +68,7 @@ import subprocess
 import tempfile
 import bb
 import bb.progress
+from contextlib import contextmanager
 from   bb.fetch2 import FetchMethod
 from   bb.fetch2 import runfetchcmd
 from   bb.fetch2 import logger
@@ -418,6 +419,20 @@ class Git(FetchMethod):
                 bb.utils.remove(tmpdir, recurse=True)
 
     def build_mirror_data(self, ud, d):
+
+        # Create as a temp file and move atomically into position to avoid races
+        @contextmanager
+        def create_atomic(filename, d):
+            fd, tfile = tempfile.mkstemp(dir=os.path.dirname(filename))
+            try:
+                yield tfile
+                umask = os.umask(0o666)
+                os.umask(umask)
+                os.chmod(tfile, (0o666 & ~umask))
+                runfetchcmd("mv %s %s" % (tfile, filename), d)
+            finally:
+                os.close(fd)
+
         if ud.shallow and ud.write_shallow_tarballs:
             if not os.path.exists(ud.fullshallow):
                 if os.path.islink(ud.fullshallow):
@@ -428,7 +443,8 @@ class Git(FetchMethod):
                     self.clone_shallow_local(ud, shallowclone, d)
 
                     logger.info("Creating tarball of git repository")
-                    runfetchcmd("tar -czf %s ." % ud.fullshallow, d, workdir=shallowclone)
+                    with create_atomic(ud.fullshallow, d) as tfile:
+                        runfetchcmd("tar -czf %s ." % tfile, d, workdir=shallowclone)
                     runfetchcmd("touch %s.done" % ud.fullshallow, d)
                 finally:
                     bb.utils.remove(tempdir, recurse=True)
@@ -437,7 +453,8 @@ class Git(FetchMethod):
                 os.unlink(ud.fullmirror)
 
             logger.info("Creating tarball of git repository")
-            runfetchcmd("tar -czf %s ." % ud.fullmirror, d, workdir=ud.clonedir)
+            with create_atomic(ud.fullmirror, d) as tfile:
+                runfetchcmd("tar -czf %s ." % tfile, d, workdir=ud.clonedir)
             runfetchcmd("touch %s.done" % ud.fullmirror, d)
 
     def clone_shallow_local(self, ud, dest, d):