tools, github: Add current SOURCE_VERSION writer

tools/github/write_current_source_version.py

@@ -0,0 +1,49 @@
+# This script produces SOURCE_VERSION file with content from current version commit hash. As a
+# reminder,SOURCE_VERSION is required when building Envoy from an extracted release tarball
+# (non-git). See: bazel/get_workspace_status for more information.
+#
+# The SOURCE_VERSION file is produced by reading current version tag from VERSION.txt file then
+# fetch the corresponding commit hash from GitHub.
+#
+# Note: This script can only be executed from project root directory of an extracted "release"
+# tarball.
+
+import sys
+import json
+
+from urllib import request
+from pathlib import Path
+
+# Simple check if a .git directory exists. When we are in a Git repo, we should rely on git.
+if Path(".git").exists():
+    print(
+        "Failed to create SOURCE_VERSION. "
+        "Run this script from an extracted release tarball directory.")
+    sys.exit(1)
+
+# Check if we have VERSION.txt available
+current_version_file = Path("VERSION.txt")
+if not current_version_file.exists():
+    print(
+        "Failed to read VERSION.txt. "
+        "Run this script from project root of an extracted release tarball directory.")
+    sys.exit(1)
+
+current_version = current_version_file.read_text().rstrip()
+
+# Exit when we are in a "main" copy.
+if current_version.endswith("-dev"):
+    print(
+        "Failed to create SOURCE_VERSION. "
+        "The current VERSION.txt contains version with '-dev' suffix. "
+        "Run this script from an extracted release tarball directory.")
+    sys.exit(1)
+
+# Fetch the current version commit information from GitHub.
+with request.urlopen("https://api.github.com/repos/envoyproxy/envoy/commits/v"
+                     + current_version) as response:
+    commit_info = json.loads(response.read())
+    source_version_file = Path("SOURCE_VERSION")
+    with source_version_file.open("w", encoding="utf-8") as source_version:
+        # Write the extracted current version commit hash "sha" to SOURCE_VERSION.
+        source_version.write(commit_info["sha"])