copy_file: Add parameter to allow symlinks

This change adds a new parameter `allow_symlinks` to `copy_file` that
allows the action to create a symlink instead of doing an expensive
copy if the execution platform (host) allows it.

Updates #248

rules/private/copy_file_private.bzl

@@ -59,10 +59,17 @@ def copy_bash(ctx, src, dst):
     )
 
 def _common_impl(ctx, is_executable):
-    if ctx.attr.is_windows:
-        copy_cmd(ctx, ctx.file.src, ctx.outputs.out)
+    if ctx.attr.allow_symlink:
+        ctx.actions.symlink(
+            output = ctx.outputs.out,
+            target_file = ctx.file.src,
+            is_executable = is_executable,
+        )
     else:
-        copy_bash(ctx, ctx.file.src, ctx.outputs.out)
+        if ctx.attr.is_windows:
+            copy_cmd(ctx, ctx.file.src, ctx.outputs.out)
+        else:
+            copy_bash(ctx, ctx.file.src, ctx.outputs.out)
 
     files = depset(direct = [ctx.outputs.out])
     runfiles = ctx.runfiles(files = [ctx.outputs.out])
@@ -81,6 +88,7 @@ _ATTRS = {
     "src": attr.label(mandatory = True, allow_single_file = True),
     "out": attr.output(mandatory = True),
     "is_windows": attr.bool(mandatory = True),
+    "allow_symlink": attr.bool(mandatory = True),
 }
 
 _copy_file = rule(
@@ -96,7 +104,7 @@ _copy_xfile = rule(
     attrs = _ATTRS,
 )
 
-def copy_file(name, src, out, is_executable = False, **kwargs):
+def copy_file(name, src, out, is_executable = False, allow_symlink = False, **kwargs):
     """Copies a file to another location.
 
     `native.genrule()` is sometimes used to copy files (often wishing to rename them). The 'copy_file' rule does this with a simpler interface than genrule.
@@ -111,6 +119,13 @@ def copy_file(name, src, out, is_executable = False, **kwargs):
       is_executable: A boolean. Whether to make the output file executable. When
           True, the rule's output can be executed using `bazel run` and can be
           in the srcs of binary and test rules that require executable sources.
+          WARNING: If `allow_symlink` is True, `src` must also be executable.
+      allow_symlink: A boolean. Whether to allow symlinking instead of copying.
+          When False, the output is always a hard copy. When True, the output
+          *can* be a symlink, but there is no guarantee that a symlink is
+          created (i.e., at the time of writing, we don't create symlinks on
+          Windows). Set this to True if you need fast copying and your tools can
+          handle symlinks (which most UNIX tools can).
       **kwargs: further keyword arguments, e.g. `visibility`
     """
     if is_executable:
@@ -122,6 +137,7 @@ def copy_file(name, src, out, is_executable = False, **kwargs):
                 "@bazel_tools//src/conditions:host_windows": True,
                 "//conditions:default": False,
             }),
+            allow_symlink = allow_symlink,
             **kwargs
         )
     else:
@@ -133,5 +149,6 @@ def copy_file(name, src, out, is_executable = False, **kwargs):
                 "@bazel_tools//src/conditions:host_windows": True,
                 "//conditions:default": False,
             }),
+            allow_symlink = allow_symlink,
             **kwargs
         )

tests/copy_file/BUILD

@@ -46,6 +46,7 @@ sh_test(
         ":file_deps",
         # Use DefaultInfo.runfiles from 'copy_gen'.
         ":copy_gen",
+        ":copy_gen_symlink",
         "//tests:unittest.bash",
     ],
     deps = ["@bazel_tools//tools/bash/runfiles"],
@@ -56,6 +57,7 @@ filegroup(
     # Use DefaultInfo.files from 'copy_src'.
     srcs = [
         ":copy_src",
+        ":copy_src_symlink",
     ],
 )
 
@@ -64,15 +66,23 @@ filegroup(
 genrule(
     name = "run_executables",
     outs = [
+        "xsrc-out-symlink.txt",
+        "xgen-out-symlink.txt",
         "xsrc-out.txt",
         "xgen-out.txt",
     ],
-    cmd = ("$(location :bin_src) > $(location xsrc-out.txt) && " +
-           "$(location :bin_gen) > $(location xgen-out.txt)"),
+    cmd = " && ".join([
+        "$(location :bin_src_symlink) > $(location xsrc-out-symlink.txt)",
+        "$(location :bin_gen_symlink) > $(location xgen-out-symlink.txt)",
+        "$(location :bin_src) > $(location xsrc-out.txt)",
+        "$(location :bin_gen) > $(location xgen-out.txt)",
+    ]),
     output_to_bindir = 1,
     tools = [
         ":bin_gen",
         ":bin_src",
+        ":bin_gen_symlink",
+        ":bin_src_symlink",
     ],
 )
 
@@ -82,22 +92,48 @@ sh_binary(
     srcs = [":copy_xsrc"],
 )
 
+# If 'bin_src' is built, then 'copy_xsrc' made its output executable.
+sh_binary(
+    name = "bin_src_symlink",
+    srcs = [":copy_xsrc_symlink"],
+)
+
 # If 'bin_gen' is built, then 'copy_xgen' made its output executable.
 sh_binary(
     name = "bin_gen",
     srcs = [":copy_xgen"],
 )
 
+# If 'bin_gen' is built, then 'copy_xgen' made its output executable.
+sh_binary(
+    name = "bin_gen_symlink",
+    srcs = [":copy_xgen_symlink"],
+)
+
 copy_file(
     name = "copy_src",
     src = "a.txt",
     out = "out/a-out.txt",
 )
 
+copy_file(
+    name = "copy_src_symlink",
+    src = "a.txt",
+    out = "out/a-out-symlink.txt",
+    allow_symlink = True,
+)
+
 copy_file(
     name = "copy_gen",
     src = ":gen",
     out = "out/gen-out.txt",
+    allow_symlink = True,
+)
+
+copy_file(
+    name = "copy_gen_symlink",
+    src = ":gen",
+    out = "out/gen-out-symlink.txt",
 )
 
 copy_file(
@@ -107,13 +143,29 @@ copy_file(
     is_executable = True,
 )
 
+copy_file(
+    name = "copy_xsrc_symlink",
+    src = "a_with_exec_bit.txt",
+    out = "xout/a-out-symlink.sh",
+    is_executable = True,
+    allow_symlink = True,
+)
+
 copy_file(
     name = "copy_xgen",
     src = ":gen",
     out = "xout/gen-out.sh",
     is_executable = True,
 )
 
+copy_file(
+    name = "copy_xgen_symlink",
+    src = ":gen",
+    out = "xout/gen-out-symlink.sh",
+    is_executable = True,
+    allow_symlink = True,
+)
+
 genrule(
     name = "gen",
     outs = ["b.txt"],

tests/copy_file/a_with_exec_bit.txt

@@ -0,0 +1,2 @@
+#!/bin/bash
+echo aaa

tests/copy_file/copy_file_tests.sh

@@ -44,20 +44,42 @@ function test_copy_src() {
   expect_log '^echo aaa$'
 }
 
+function test_copy_src_symlink() {
+  cat "$(rlocation bazel_skylib/tests/copy_file/out/a-out-symlink.txt)" >"$TEST_log"
+  expect_log '^#!/bin/bash$'
+  expect_log '^echo aaa$'
+}
+
 function test_copy_gen() {
   cat "$(rlocation bazel_skylib/tests/copy_file/out/gen-out.txt)" >"$TEST_log"
   expect_log '^#!/bin/bash$'
   expect_log '^echo potato$'
 }
 
+function test_copy_gen_symlink() {
+  cat "$(rlocation bazel_skylib/tests/copy_file/out/gen-out-symlink.txt)" >"$TEST_log"
+  expect_log '^#!/bin/bash$'
+  expect_log '^echo potato$'
+}
+
 function test_copy_xsrc() {
   cat "$(rlocation bazel_skylib/tests/copy_file/xsrc-out.txt)" >"$TEST_log"
   expect_log '^aaa$'
 }
 
+function test_copy_xsrc_symlink() {
+  cat "$(rlocation bazel_skylib/tests/copy_file/xsrc-out-symlink.txt)" >"$TEST_log"
+  expect_log '^aaa$'
+}
+
 function test_copy_xgen() {
   cat "$(rlocation bazel_skylib/tests/copy_file/xgen-out.txt)" >"$TEST_log"
   expect_log '^potato$'
 }
 
+function test_copy_xgen_symlink() {
+  cat "$(rlocation bazel_skylib/tests/copy_file/xgen-out-symlink.txt)" >"$TEST_log"
+  expect_log '^potato$'
+}
+
 run_suite "copy_file_tests test suite"