Override the full Rust+Clang version in FALLBACK_REVISION

The fallback mechanism was designed for when we were rolling Rust
separately from Clang. As such, the Rust revision would never bump
until we had a successful build.

Now we are rolling Rust and Clang together, but we're not yet blocking
the Clang roll on Rust succeeding. This means the Rust revision
changes but doesn't refer to a successful build. Then the fallback
revision tries to use it and can't find the toolchain.

Until we block the Clang roll on Rust, have the rust fallback path
specify and pin to the full Rust + Clang versions.

Bug: 1409334
Change-Id: I51aa792ab3a5349fae94e04becd23b9d0b344f7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4186736
Auto-Submit: danakj <[email protected]>
Reviewed-by: Adrian Taylor <[email protected]>
Commit-Queue: Adrian Taylor <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1095496}
NOKEYCHECK=True
GitOrigin-RevId: 7ba24de9daf83eca666514391f17176f84f0133b

Author: danakj

update_rust.py

@@ -51,12 +51,16 @@
 #
 # This should almost always be None. When a breakage happens the fallback should
 # be temporary. Once fixed, the applicable revision(s) above should be updated
-# and FALLBACK_CLANG_VERSION should be reset to None.
+# and FALLBACK_REVISION should be reset to None.
 #
 # Rust builds (for Linux) that worked are found at:
 # https://commondatastorage.googleapis.com/chromium-browser-clang/index.html?path=Linux_x64/rust-toolchain-
 # The latest builds are prefixed with a date, such as `20230101-1`.
-FALLBACK_CLANG_VERSION = 'llvmorg-16-init-13328-g110fe4f4-1'
+#
+# TODO(lukasza): Include CRUBIT_REVISION and CRUBIT_SUB_REVISION once we
+# include Crubit binaries in the generated package.  See also a TODO comment
+# in BuildCrubit in package_rust.py.
+FALLBACK_REVISION = '20221209-1-llvmorg-16-init-13328-g110fe4f4-1'
 
 # Hash of src/stage0.json, which itself contains the stage0 toolchain hashes.
 # We trust the Rust build system checks, but to ensure it is not tampered with
@@ -70,15 +74,6 @@
 VERSION_STAMP_PATH = os.path.join(RUST_TOOLCHAIN_OUT_DIR, 'VERSION')
 
 
-# Get the package version for the RUST_[SUB_]REVISION above with the specified
-# clang_version.
-def GetPackageVersion(clang_version):
-    # TODO(lukasza): Include CRUBIT_REVISION and CRUBIT_SUB_REVISION once we
-    # include Crubit binaries in the generated package.  See also a TODO comment
-    # in BuildCrubit in package_rust.py.
-    return f'{RUST_REVISION}-{RUST_SUB_REVISION}-{clang_version}'
-
-
 # Package version built in build_rust.py, which is always built against the
 # latest Clang and never uses the FALLBACK_CLANG_VERSION.
 def GetPackageVersionForBuild():
@@ -89,10 +84,8 @@ def GetPackageVersionForBuild():
 # Package version for download, which may differ from GetUploadPackageVersion()
 # if FALLBACK_CLANG_VERSION is set.
 def GetDownloadPackageVersion():
-    if FALLBACK_CLANG_VERSION:
-        return GetPackageVersion(FALLBACK_CLANG_VERSION)
-    else:
-        return GetPackageVersionForBuild()
+    return FALLBACK_REVISION \
+        if FALLBACK_REVISION else GetPackageVersionForBuild()
 
 
 # Get the version of the toolchain package we already have.