Allow for requirements files to differ per platform

As a common example, we need a compiled requirements file for linux that differs from mac os

Author: Alex Eagle

docs/pip.md

@@ -140,7 +140,7 @@ alias(
 
 | Name  | Description | Default Value |
 | :-------------: | :-------------: | :-------------: |
-| requirements |  A 'requirements.txt' pip requirements file.   |  none |
+| requirements |  A 'requirements.txt' pip requirements file.   |  <code>None</code> |
 | name |  A unique name for the created external repository (default 'pip').   |  <code>"pip"</code> |
 | kwargs |  Keyword arguments passed directly to the <code>pip_repository</code> repository rule.   |  none |
 

python/pip.bzl

@@ -20,7 +20,7 @@ load("//python/pip_install:requirements.bzl", _compile_pip_requirements = "compi
 compile_pip_requirements = _compile_pip_requirements
 package_annotation = _package_annotation
 
-def pip_install(requirements, name = "pip", **kwargs):
+def pip_install(requirements = None, name = "pip", **kwargs):
     """Accepts a `requirements.txt` file and installs the dependencies listed within.
 
     Those dependencies become available in a generated `requirements.bzl` file.

python/pip_install/pip_repository.bzl

@@ -122,8 +122,19 @@ exports_files(["requirements.bzl"])
 def _pip_repository_impl(rctx):
     python_interpreter = _resolve_python_interpreter(rctx)
 
-    if rctx.attr.incremental and not rctx.attr.requirements_lock:
-        fail("Incremental mode requires a requirements_lock attribute be specified.")
+    os = rctx.os.name.lower()
+    requirements_txt = rctx.attr.requirements_lock
+    if os.startswith("mac os") and rctx.attr.requirements_darwin != None:
+        requirements_txt = rctx.attr.requirements_darwin
+    elif os.startswith("linux") and rctx.attr.requirements_linux != None:
+        requirements_txt = rctx.attr.requirements_linux
+    elif "win" in os:
+        requirements_txt = rctx.attr.requirements_windows
+    if rctx.attr.incremental and not requirements_txt:
+        fail("""\
+Incremental mode requires a requirements_lock attribute be specified,
+or a platform-specific lockfile using one of the requirements_* attributes.
+""")
 
     # Write the annotations file to pass to the wheel maker
     annotations = {package: json.decode(data) for (package, data) in rctx.attr.annotations.items()}
@@ -136,9 +147,9 @@ def _pip_repository_impl(rctx):
             "-m",
             "python.pip_install.parse_requirements_to_bzl",
             "--requirements_lock",
-            rctx.path(rctx.attr.requirements_lock),
+            rctx.path(requirements_txt),
             "--requirements_lock_label",
-            str(rctx.attr.requirements_lock),
+            str(requirements_txt),
             # pass quiet and timeout args through to child repos.
             "--quiet",
             str(rctx.attr.quiet),
@@ -158,7 +169,7 @@ def _pip_repository_impl(rctx):
             "-m",
             "python.pip_install.extract_wheels",
             "--requirements",
-            rctx.path(rctx.attr.requirements),
+            rctx.path(requirements_txt),
             "--annotations",
             annotations_file,
         ]
@@ -282,6 +293,18 @@ pip_repository_attrs = {
         allow_single_file = True,
         doc = "A 'requirements.txt' pip requirements file.",
     ),
+    "requirements_darwin": attr.label(
+        allow_single_file = True,
+        doc = "Override the requirements_lock attribute when the host platform is Mac OS",
+    ),
+    "requirements_linux": attr.label(
+        allow_single_file = True,
+        doc = "Override the requirements_lock attribute when the host platform is Linux",
+    ),
+    "requirements_windows": attr.label(
+        allow_single_file = True,
+        doc = "Override the requirements_lock attribute when the host platform is Windows",
+    ),
     "requirements_lock": attr.label(
         allow_single_file = True,
         doc = """