Add alternative method to use a version/git hash from a file.

- This is necessary for Chromium builds from tarballs where git is no longer available. This gives Chromium another option to create the version file when creating the tarball to accomplish the same thing.

Bug: dawn:549
Change-Id: Iffb4bf694b0df1306dd92939353422e5115346a7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94043
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

generator/dawn_version_generator.py

@@ -26,11 +26,11 @@ def get_git():
     return git_exec
 
 
-def get_gitHash(dawnDir):
+def get_git_hash(dawn_dir):
     try:
         result = subprocess.run([get_git(), "rev-parse", "HEAD"],
                                 stdout=subprocess.PIPE,
-                                cwd=dawnDir)
+                                cwd=dawn_dir)
         if result.returncode == 0:
             return result.stdout.decode("utf-8").strip()
     except Exception:
@@ -40,15 +40,15 @@ def get_gitHash(dawnDir):
     return ""
 
 
-def get_gitHead(dawnDir):
-    return os.path.join(dawnDir, ".git", "HEAD")
+def get_git_head(dawn_dir):
+    return os.path.join(dawn_dir, ".git", "HEAD")
 
 
-def gitExists(dawnDir):
-    return os.path.exists(get_gitHead(dawnDir))
+def git_exists(dawn_dir):
+    return os.path.exists(get_git_head(dawn_dir))
 
 
-def unpackGitRef(packed, resolved):
+def unpack_git_ref(packed, resolved):
     with open(packed) as fin:
         refs = fin.read().strip().split("\n")
 
@@ -64,20 +64,20 @@ def unpackGitRef(packed, resolved):
     return False
 
 
-def get_gitResolvedHead(dawnDir):
+def get_git_resolved_head(dawn_dir):
     result = subprocess.run(
         [get_git(), "rev-parse", "--symbolic-full-name", "HEAD"],
         stdout=subprocess.PIPE,
-        cwd=dawnDir)
+        cwd=dawn_dir)
     if result.returncode != 0:
         raise Exception("Failed to execute git rev-parse to resolve git head:", result.stdout)
 
-    resolved = os.path.join(dawnDir, ".git",
+    resolved = os.path.join(dawn_dir, ".git",
                             result.stdout.decode("utf-8").strip())
 
     # Check a packed-refs file exists. If so, we need to potentially unpack and include it as a dep.
-    packed = os.path.join(dawnDir, ".git", "packed-refs")
-    if os.path.exists(packed) and unpackGitRef(packed, resolved):
+    packed = os.path.join(dawn_dir, ".git", "packed-refs")
+    if os.path.exists(packed) and unpack_git_ref(packed, resolved):
         return [packed, resolved]
 
     if not os.path.exists(resolved):
@@ -85,15 +85,25 @@ def get_gitResolvedHead(dawnDir):
     return [resolved]
 
 
+def get_version(args):
+    version_file = args.version_file
+    if version_file:
+        with open(version_file) as f:
+            return f.read()
+    return get_git_hash(os.path.abspath(args.dawn_dir))
+
+
 def compute_params(args):
     return {
-        "get_gitHash": lambda: get_gitHash(os.path.abspath(args.dawn_dir)),
+        "get_version": lambda: get_version(args),
     }
 
 
 class DawnVersionGenerator(Generator):
     def get_description(self):
-        return "Generates version dependent Dawn code. Currently regenerated dependent on git hash."
+        return (
+            "Generates version dependent Dawn code. Currently regenerated dependent on the version "
+            "header (if available), otherwise tries to use git hash.")
 
     def add_commandline_arguments(self, parser):
         parser.add_argument(
@@ -102,15 +112,28 @@ class DawnVersionGenerator(Generator):
             type=str,
             help="The Dawn root directory path to use",
         )
+        parser.add_argument(
+            "--version-file",
+            required=False,
+            type=str,
+            help=
+            ("Path to one-liner version string file used when git may not be present. "
+             "In general the version string is a git hash."))
 
     def get_dependencies(self, args):
-        dawnDir = os.path.abspath(args.dawn_dir)
-        if gitExists(dawnDir):
+        dawn_dir = os.path.abspath(args.dawn_dir)
+        version_file = args.version_file
+
+        if version_file:
+            return [version_file]
+        if git_exists(dawn_dir):
+            deps = []
             try:
-                return [get_gitHead(dawnDir)] + get_gitResolvedHead(dawnDir)
+                deps += [get_git_head(dawn_dir)
+                         ] + get_git_resolved_head(dawn_dir)
             except Exception:
-                return []
-        return []
+                return deps
+        return deps
 
     def get_file_renders(self, args):
         params = compute_params(args)