Add abs_ireq for normalizing ireqs, and related tests

Tests:

- abs_ireq_preserves_source_ireqs
- format_requirement_impossible_relative_path_becomes_absolute (Windows-only)

Also, ensure copy_install_requirement preserves our custom attributes.

piptools/utils.py

@@ -3,6 +3,7 @@
 import itertools
 import json
 import os
+import platform
 import re
 import shlex
 from contextlib import contextmanager
@@ -23,10 +24,11 @@
 
 import click
 from click.utils import LazyFile
+from pip._internal.models.link import Link
 from pip._internal.req import InstallRequirement
 from pip._internal.req.constructors import install_req_from_line
 from pip._internal.utils.misc import redact_auth_from_url
-from pip._internal.utils.urls import path_to_url
+from pip._internal.utils.urls import path_to_url, url_to_path
 from pip._internal.vcs import is_url
 from pip._vendor.packaging.markers import Marker
 from pip._vendor.packaging.specifiers import SpecifierSet
@@ -367,6 +369,52 @@ def working_dir(folder: Optional[str]) -> Iterator[None]:
             os.chdir(original_dir)
 
 
+def abs_ireq(
+    ireq: InstallRequirement, from_dir: Optional[str] = None
+) -> InstallRequirement:
+    """
+    Return the given InstallRequirement if its source isn't a relative path;
+    Otherwise, return a new one with the relative path rewritten as absolute.
+
+    In this case, an extra attribute is added: _was_relative,
+    which is always True when present at all.
+    """
+    # We check ireq.link.scheme rather than ireq.link.is_file,
+    # to also match <vcs>+file schemes
+    if ireq.link is None or not ireq.link.scheme.endswith("file"):
+        return ireq
+
+    naive_path = ireq.local_file_path or ireq.link.path
+    if platform.system() == "Windows":
+        naive_path = naive_path.lstrip("/")
+
+    with working_dir(from_dir):
+        url = path_to_url(naive_path).replace("%40", "@")
+
+    if (
+        os.path.normpath(naive_path).lower()
+        == os.path.normpath(url_to_path(url)).lower()
+    ):
+        return ireq
+
+    abs_url = f"{url}{fragment_string(ireq)}"
+    if "+" in ireq.link.scheme:
+        abs_url = f"{ireq.link.scheme.split('+')[0]}+{abs_url}"
+
+    abs_link = Link(
+        url=abs_url,
+        comes_from=ireq.link.comes_from,
+        requires_python=ireq.link.requires_python,
+        yanked_reason=ireq.link.yanked_reason,
+        cache_link_parsing=ireq.link.cache_link_parsing,
+    )
+
+    a_ireq = copy_install_requirement(ireq, link=abs_link)
+    a_ireq._was_relative = True
+
+    return a_ireq
+
+
 def get_compile_command(click_ctx: click.Context) -> str:
     """
     Returns a normalized compile command depending on cli context.
@@ -540,4 +588,8 @@ def copy_install_requirement(
         template.original_link if original_link is None else original_link
     )
 
+    for custom_attr in ("_source_ireqs", "_was_relative"):
+        if hasattr(template, custom_attr):
+            setattr(ireq, custom_attr, getattr(template, custom_attr))
+
     return ireq

tests/test_utils.py

@@ -1,6 +1,7 @@
 import logging
 import operator
 import os
+import platform
 import shlex
 import sys
 
@@ -10,6 +11,7 @@
 
 from piptools.scripts.compile import cli as compile_cli
 from piptools.utils import (
+    abs_ireq,
     as_tuple,
     dedup,
     drop_extras,
@@ -145,6 +147,15 @@ def test_format_requirement_editable_local_path(from_editable):
     assert format_requirement(ireq) == "-e file:///home/user/package"
 
 
+@pytest.mark.skipif(
+    platform.system() != "Windows",
+    reason="Relative paths can only be impossible on Windows",
+)
+def test_format_requirement_impossible_relative_path_becomes_absolute(from_line):
+    ireq = abs_ireq(from_line("file:./vendor/package.zip"), ".")
+    assert format_requirement(ireq, from_dir="z:") == ireq.link.url
+
+
 def test_format_requirement_ireq_with_hashes(from_line):
     ireq = from_line("pytz==2017.2")
     ireq_hashes = [
@@ -560,3 +571,17 @@ def test_working_dir(tmpdir_cwd, folder_name, use_abspath):
         assert os.getcwd() == expected_within
 
     assert os.getcwd() == tmpdir_cwd
+
+
+def test_local_abs_ireq_preserves_source_ireqs(from_line):
+    ireq1 = from_line("testone==1.2")
+    ireq1.comes_from = "xyz"
+
+    ireq2 = from_line("testtwo==2.4")
+    ireq2.comes_from = "lmno"
+
+    ireq3 = from_line("file:testwithsrcs")
+    ireq3._source_ireqs = [ireq1, ireq2]
+    ireq3 = abs_ireq(ireq3, os.getcwd())
+
+    assert ireq3._source_ireqs == [ireq1, ireq2]