[RFC] git: add BB_FETCH_PREMIRROR_READONLY option

[Martin Jansa <martin.jansa@gmail.com>]

* just RFC, because I haven't even tested this change,
  use it only to describe the issue and discuss it

* remove symlink to ud.fullmirror when BB_FETCH_PREMIRROR_READONLY is set

* with read-only PREMIRROR (e.g. mounted over NFS or CIFS
  and referenced as file:///mnt/premirror) we cannot use
  BB_GENERATE_MIRROR_TARBALLS because all git2_abc.git.tar.gz
  files later became just symlinks to read-only location in PREMIRROR
  (it works fine on first build and for new components, because
  at that time there isn't tarball on PREMIRROR yet).

  ERROR: Fetcher failure: Fetch command failed with exit code 141, output:
  tar (child): /build/downloads/git2_abc.git.tar.gz: Cannot open: Read-only file system
  tar (child): Error is not recoverable: exiting now

* maybe we can change the default behavior and always remove symlink
  without introducing new option

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
---
 lib/bb/fetch2/git.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/bb/fetch2/git.py b/lib/bb/fetch2/git.py
index 052802e..14a8bc2 100644
--- a/lib/bb/fetch2/git.py
+++ b/lib/bb/fetch2/git.py
@@ -217,6 +217,10 @@ class Git(FetchMethod):
     def build_mirror_data(self, url, ud, d):
         # Generate a mirror tarball if needed
         if ud.write_tarballs and (ud.repochanged or not os.path.exists(ud.fullmirror)):
+            premirror_readonly = (d.getVar("BB_FETCH_PREMIRROR_READONLY", True) == "1")
+            if premirror_readonly and os.path.islink(ud.fullmirror):
+                os.unlink(origud.localpath)
+
             os.chdir(ud.clonedir)
             logger.info("Creating tarball of git repository")
             runfetchcmd("tar -czf %s %s" % (ud.fullmirror, os.path.join(".") ), d)
-- 
1.8.2.1