build: fix rust temp file conflicts during parallel build

.appveyor.yml

@@ -306,12 +306,6 @@ before_build:
       Set-FilesNeeded -Auto -Path $outputs -Reason "Build dependency graph"
 
 build_script:
-  # Attempt to work around multiple rustc instances messing with the same file
-  # when building in parallel.
-  # TODO: fix this properly.
-  - ps: ninja -C $env:DENO_BUILD_PATH -j 1
-              build_extra/rust:winapi build_extra/rust:winapi-0.2
-
   - python tools\build.py
   - ps: Set-FilesNeeded -Auto -Reason "Build finished"
 

build_extra/rust/get_version_hash.py

@@ -0,0 +1,14 @@
+# Copyright 2018 the Deno authors. All rights reserved. MIT license.
+
+# This script computes the sha256sum of the first command line argument, and
+# writes a few hex digits of it to stdout. It is used by rust.gni to derive a
+# unique string (without dots/special characters) from a crate version number.
+
+from hashlib import sha256
+import sys
+
+if len(sys.argv) != 2:
+    raise Exception('Expected exactly one argument.')
+
+hash = sha256(sys.argv[1]).hexdigest()
+sys.stdout.write(hash[0:8])

build_extra/rust/rust.gni

@@ -118,9 +118,30 @@ template("run_rustc") {
     ]
 
     if (defined(crate_version)) {
+      # Compute the sha256sum of the version number. See comments below.
+      # Note that we do this only if there are multiple versions of this crate.
+      hash =
+          exec_script("get_version_hash.py", [ crate_version ], "trim string")
+
       args += [
+        # In our build setup, all crates are built in the same directory. The
+        # actual build outputs have unique names (e.g. 'foo-1.2.3.rlib'), but
+        # rustc also creates many temp files (e.g. 'foo.crate.metadata.rcgu.bc',
+        # 'foo.foo0.rcgu.o'), and with those files, name collisions do occur.
+        # This causes random failures during parallel builds and on CI.
+        #
+        # These name conflicts can be avoided by setting `-C extra-filename=` to
+        # some unique value. Unfortunately the version number as such can't be
+        # used: everything after the first dot (.) is thrown away, so winapi-0.2
+        # vs. winapi-0.3 would still have conflicts -- so we use a hash instead.
         "-C",
-        "metadata=$crate_version",
+        "extra-filename=$hash",
+
+        # Rustc blows up if a target (directly or indirectly) depends on two+
+        # crates that have the same name *and* the same metadata. So we use the
+        # hash to give 'metadata' a unique value too (any unique value will do).
+        "-C",
+        "metadata=$hash",
       ]
     }