Bazel: allow LFS rules to use cached downloads without internet

If the cache is prefilled, LFS rules were still trying to query LFS
urls.

Now the strategy is to first try to fetch the files from the repository
cache (which is possible by providing an empty url list and `allow_fail`
to `repository_ctx.download`), and only run the LFS protocol if that
fails. Technically this is possible by enhancing `git_lfs_probe.py` with
a `--hash-only` flag.

This is also an optimization where no uneeded access is done (including
the slightly slow SSH call) if the repository cache is warm.

misc/bazel/internal/git_lfs_probe.py

@@ -2,9 +2,10 @@
 
 """
 Probe lfs files.
-For each source file provided as output, this will print:
+For each source file provided as input, this will print:
 * "local", if the source file is not an LFS pointer
 * the sha256 hash, a space character and a transient download link obtained via the LFS protocol otherwise
+If --hash-only is provided, the transient URL will not be fetched and printed
 """
 
 import sys
@@ -19,6 +20,13 @@ import re
 import base64
 from dataclasses import dataclass
 from typing import Dict
+import argparse
+
+def options():
+    p = argparse.ArgumentParser(description=__doc__)
+    p.add_argument("--hash-only", action="store_true")
+    p.add_argument("sources", type=pathlib.Path, nargs="+")
+    return p.parse_args()
 
 
 @dataclass
@@ -30,7 +38,8 @@ class Endpoint:
         self.headers.update((k.capitalize(), v) for k, v in d.items())
 
 
-sources = [pathlib.Path(arg).resolve() for arg in sys.argv[1:]]
+opts = options()
+sources = [p.resolve() for p in opts.sources]
 source_dir = pathlib.Path(os.path.commonpath(src.parent for src in sources))
 source_dir = subprocess.check_output(["git", "rev-parse", "--show-toplevel"], cwd=source_dir, text=True).strip()
 
@@ -84,11 +93,15 @@ def get_endpoint():
 # see https://github.com/git-lfs/git-lfs/blob/310d1b4a7d01e8d9d884447df4635c7a9c7642c2/docs/api/basic-transfers.md
 def get_locations(objects):
     ret = ["local" for _ in objects]
-    endpoint = get_endpoint()
     indexes = [i for i, o in enumerate(objects) if o]
     if not indexes:
         # all objects are local, do not send an empty request as that would be an error
         return ret
+    if opts.hash_only:
+        for i in indexes:
+            ret[i] = objects[i]["oid"]
+        return ret
+    endpoint = get_endpoint()
     data = {
         "operation": "download",
         "transfers": ["basic"],

misc/bazel/lfs.bzl

@@ -1,36 +1,44 @@
 def lfs_smudge(repository_ctx, srcs, extract = False, stripPrefix = None):
-    for src in srcs:
-        repository_ctx.watch(src)
-    script = Label("//misc/bazel/internal:git_lfs_probe.py")
     python = repository_ctx.which("python3") or repository_ctx.which("python")
     if not python:
         fail("Neither python3 nor python executables found")
-    repository_ctx.report_progress("querying LFS url(s) for: %s" % ", ".join([src.basename for src in srcs]))
-    res = repository_ctx.execute([python, script] + srcs, quiet = True)
-    if res.return_code != 0:
-        fail("git LFS probing failed while instantiating @%s:\n%s" % (repository_ctx.name, res.stderr))
-    promises = []
-    for src, loc in zip(srcs, res.stdout.splitlines()):
-        if loc == "local":
-            if extract:
-                repository_ctx.report_progress("extracting local %s" % src.basename)
-                repository_ctx.extract(src, stripPrefix = stripPrefix)
-            else:
-                repository_ctx.report_progress("symlinking local %s" % src.basename)
-                repository_ctx.symlink(src, src.basename)
+    script = Label("//misc/bazel/internal:git_lfs_probe.py")
+
+    def probe(srcs, hash_only = False):
+        repository_ctx.report_progress("querying LFS url(s) for: %s" % ", ".join([src.basename for src in srcs]))
+        cmd = [python, script]
+        if hash_only:
+            cmd.append("--hash-only")
+        cmd.extend(srcs)
+        res = repository_ctx.execute(cmd, quiet = True)
+        if res.return_code != 0:
+            fail("git LFS probing failed while instantiating @%s:\n%s" % (repository_ctx.name, res.stderr))
+        return res.stdout.splitlines()
+
+    for src in srcs:
+        repository_ctx.watch(src)
+    infos = probe(srcs, hash_only = True)
+    remote = []
+    for src, info in zip(srcs, infos):
+        if info == "local":
+            repository_ctx.report_progress("symlinking local %s" % src.basename)
+            repository_ctx.symlink(src, src.basename)
         else:
-            sha256, _, url = loc.partition(" ")
-            if extract:
-                # we can't use skylib's `paths.split_extension`, as that only gets the last extension, so `.tar.gz`
-                # or similar wouldn't work
-                # it doesn't matter if file is something like some.name.zip and possible_extension == "name.zip",
-                # download_and_extract will just append ".name.zip" its internal temporary name, so extraction works
-                possible_extension = ".".join(src.basename.rsplit(".", 2)[-2:])
-                repository_ctx.report_progress("downloading and extracting remote %s" % src.basename)
-                repository_ctx.download_and_extract(url, sha256 = sha256, stripPrefix = stripPrefix, type = possible_extension)
-            else:
+            repository_ctx.report_progress("trying cache for remote %s" % src.basename)
+            res = repository_ctx.download([], src.basename, sha256 = info, allow_fail = True)
+            if not res.success:
+                remote.append(src)
+        if remote:
+            infos = probe(remote)
+            for src, info in zip(remote, infos):
+                sha256, _, url = info.partition(" ")
                 repository_ctx.report_progress("downloading remote %s" % src.basename)
                 repository_ctx.download(url, src.basename, sha256 = sha256)
+        if extract:
+            for src in srcs:
+                repository_ctx.report_progress("extracting %s" % src.basename)
+                repository_ctx.extract(src.basename, stripPrefix = stripPrefix)
+                repository_ctx.delete(src.basename)
 
 def _download_and_extract_lfs(repository_ctx):
     attr = repository_ctx.attr