git: make shallow clones with explicit commits more reliable

On a shallow clone the fetched tip(s) are only partially cloned. If the
scm is configured to use a specific commit it might be possible that the
commit was not downloaded if the branch advanced too much. Add a special
fallback to re-fetch from the server with the explicit commit id if the
initial fetch failed to download the required commit.

Note that we cannot just always request the commit explicitly when
fetching. Servers are usually configured to *not* hand out arbitrary
commits.

Author: jkloetzke

pym/bob/scm/git.py

@@ -182,6 +182,22 @@ async def __checkoutTag(self, invoker, fetchCmd, switch):
             stdout=False, cwd=self.__dir)
         if head or switch:
             await invoker.checkCommand(fetchCmd, cwd=self.__dir)
+            if self.__commit and (self.__shallow is not None):
+                # Shallow clones might not fetch the requested commit if the
+                # depth is too small. The problem is that we cannot blindly
+                # always request the commit explicitly because this is rejected
+                # by git-upload-pack by default. Instead check here that we got
+                # the commit and only if this is not the case we fetch a 2nd
+                # time with the explicit commit.
+                haveCommit = await invoker.callCommand(["git", "cat-file", "-e",
+                    self.__commit], cwd=self.__dir) == 0
+                if not haveCommit:
+                    ok = await invoker.callCommand(fetchCmd + [self.__commit],
+                        cwd=self.__dir)
+                    if ok != 0:
+                        self.fail("Plain git-fetch in", self.__dir,
+                            "did not download the requested commit and the explicit fetch failed!",
+                            returncode=ok)
             await invoker.checkCommand(["git", "checkout", "-q", "--no-recurse-submodules",
                 self.__commit if self.__commit else "tags/"+self.__tag], cwd=self.__dir)
             # FIXME: will not be called again if interrupted!