git_repository: support branch as well

Now that git_repository returns the commit actually checked out
(and we have means of recording it), support following an active
branch of a git repository.

Change-Id: I6e152c59b694bbf562b345ee88282a8c9bd58e4c
PiperOrigin-RevId: 203110735

3 files changed, 92 insertions, 9 deletions

diff --git a/src/test/shell/bazel/skylark_git_repository_test.sh b/src/test/shell/bazel/skylark_git_repository_test.sh
index 3509a9bc43..4fb422bc17 100755
--- a/src/test/shell/bazel/skylark_git_repository_test.sh
+++ b/src/test/shell/bazel/skylark_git_repository_test.sh
@@ -501,7 +501,7 @@ EOF
 
   bazel fetch //planets:planet-info >& $TEST_log \
     || echo "Expect run to fail."
-  expect_log "Exactly one of commit and tag must be provided"
+  expect_log "Exactly one of commit"
 }
 
 # Verifies that rule fails if neither tag or commit are set.
@@ -528,7 +528,7 @@ EOF
 
   bazel fetch //planets:planet-info >& $TEST_log \
     || echo "Expect run to fail."
-  expect_log "Exactly one of commit and tag must be provided"
+  expect_log "Exactly one of commit"
 }
 
 # Verifies that if a non-existent subdirectory is supplied, then strip_prefix
diff --git a/src/test/shell/bazel/workspace_resolved_test.sh b/src/test/shell/bazel/workspace_resolved_test.sh
index 624a90f7fc..e3a18f21c2 100755
--- a/src/test/shell/bazel/workspace_resolved_test.sh
+++ b/src/test/shell/bazel/workspace_resolved_test.sh
@@ -166,6 +166,78 @@ EOF
        && fail "not taking the frozen commit" || :
 }
 
+test_git_follow_branch() {
+  EXTREPODIR=`pwd`
+  export GIT_CONFIG_NOSYSTEM=YES
+
+  mkdir extgit
+  (cd extgit && git init \
+       && git config user.email 'me@example.com' \
+       && git config user.name 'E X Ample' )
+  echo Hello World > extgit/hello.txt
+  (cd extgit
+   git add .
+   git commit --author="A U Thor <author@example.com>" -m 'initial commit')
+  # Check out the external git repository at the given branch, and record
+  # the return value of the git rule.
+  mkdir branchcheckout
+  cd branchcheckout
+  cat > WORKSPACE <<EOF
+load("@bazel_tools//tools/build_defs/repo:git.bzl", "new_git_repository")
+new_git_repository(
+  name="ext",
+  remote="file://${EXTREPODIR}/extgit/.git",
+  branch="master",
+  build_file_content="exports_files([\"hello.txt\"])",
+)
+EOF
+  bazel sync --experimental_repository_resolved_file=../repo.bzl
+  # some of the file systems on our test machines are really slow to
+  # notice the creation of a file---even after the call to sync(1).
+  bazel shutdown; sync; sleep 10
+
+  cd ..
+  echo; cat repo.bzl; echo
+
+  # Now add an additional commit to the upstream repository
+  echo CHANGED > extgit/hello.txt
+  (cd extgit
+   git add .
+   git commit --author="A U Thor <author@example.com>" -m 'change hello.txt')
+
+  # Verify that the recorded resolved information is what we expect. In
+  # particular, verify that we don't get the new upstream commit.
+  mkdir analysisrepo
+  cd analysisrepo
+  cp ../repo.bzl .
+  cat > workspace.bzl <<'EOF'
+load("@bazel_tools//tools/build_defs/repo:git.bzl", "new_git_repository")
+load("//:repo.bzl", "resolved")
+
+def repo():
+    for entry in resolved:
+        if entry["original_attributes"]["name"] == "ext":
+            new_git_repository(**(entry["repositories"][0]["attributes"]))
+EOF
+  cat > WORKSPACE <<'EOF'
+load("//:workspace.bzl", "repo")
+repo()
+EOF
+  cat > BUILD <<'EOF'
+genrule(
+  name = "out",
+  outs = ["out.txt"],
+  srcs = ["@ext//:hello.txt"],
+  cmd = "cp $< $@",
+)
+EOF
+  bazel build //:out
+  grep "Hello World" `bazel info bazel-genfiles`/out.txt \
+       || fail "ext not taken at the right commit"
+  grep "CHANGED" `bazel info bazel-genfiles`/out.txt  \
+       && fail "not taking the frozen commit" || :
+}
+
 
 test_sync_calls_all() {
   mkdir sync_calls_all && cd sync_calls_all
diff --git a/tools/build_defs/repo/git.bzl b/tools/build_defs/repo/git.bzl
index 8b283cf9b0..2616ff6814 100644
--- a/tools/build_defs/repo/git.bzl
+++ b/tools/build_defs/repo/git.bzl
@@ -16,15 +16,19 @@
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "patch", "workspace_and_buildfile")
 
 def _clone_or_update(ctx):
-    if ((not ctx.attr.tag and not ctx.attr.commit) or
-        (ctx.attr.tag and ctx.attr.commit)):
-        fail("Exactly one of commit and tag must be provided")
+    if ((not ctx.attr.tag and not ctx.attr.commit and not ctx.attr.branch) or
+        (ctx.attr.tag and ctx.attr.commit) or
+        (ctx.attr.tag and ctx.attr.branch) or
+        (ctx.attr.commit and ctx.attr.branch)):
+        fail("Exactly one of commit, tag, or branch must be provided")
     shallow = ""
     if ctx.attr.commit:
         ref = ctx.attr.commit
-    else:
+    elif ctx.attr.tag:
         ref = "tags/" + ctx.attr.tag
         shallow = "--depth=1"
+    else:
+        ref = ctx.attr.branch
     directory = str(ctx.path("."))
     if ctx.attr.strip_prefix:
         directory = directory + "-tmp"
@@ -107,9 +111,11 @@ def _update_commit(orig, keys, override):
     result["name"] = orig.name
     result.update(override)
 
-    # remove tag if we found the actual commit
+    # if we found the actual commit, remove all other means of specifying it,
+    # like tag or branch.
     if "commit" in result:
         result.pop("tag", None)
+        result.pop("branch", None)
     return result
 
 _common_attrs = {
@@ -117,6 +123,7 @@ _common_attrs = {
     "commit": attr.string(default = ""),
     "shallow_since": attr.string(default = ""),
     "tag": attr.string(default = ""),
+    "branch": attr.string(default = ""),
     "init_submodules": attr.bool(default = False),
     "verbose": attr.bool(default = False),
     "strip_prefix": attr.string(default = ""),
@@ -181,10 +188,12 @@ Args:
 
     Either `workspace_file` or `workspace_file_content` can be specified, or
     neither, but not both.
+  branch: branch in the remote repository to checked out
+
   tag: tag in the remote repository to checked out
 
   commit: specific commit to be checked out
-    Either tag or commit must be specified.
+    Precisely one of branch, tag, or commit must be specified.
 
   shallow_since: an optional date, not after the specified commit; the
     argument is not allowed if a tag is specified (which allows cloning
@@ -225,10 +234,12 @@ Args:
 
   remote: The URI of the remote Git repository.
 
+  branch: branch in the remote repository to checked out
+
   tag: tag in the remote repository to checked out
 
   commit: specific commit to be checked out
-    Either tag or commit must be specified.
+    Precisely one of branch, tag, or commit must be specified.
 
   shallow_since: an optional date in the form YYYY-MM-DD, not after
     the specified commit; the argument is not allowed if a tag is specified