Patch harmless parts of go.mod before looking for changes in CI.

Signed-off-by: Felix Fontein <felix@fontein.de>

Author: felixfontein

patch-go.mod.py

@@ -0,0 +1,77 @@
+"""
+Patch go.mod so that the lines 'go xxx' to 'toolchain xxx' are as in git's
+HEAD.
+
+This is necessary since newer 'go mod tidy' versions tend to modify these
+lines. Since we check in CI that 'go mod tidy' does not change go.mod, this
+causes CI to fail.
+"""
+
+import subprocess
+
+
+def split_go_mod(contents: str) -> tuple[list[str], list[str], list[str]]:
+    """
+    Given the contents of go.mod, splits it into three lists of lines
+    (with endings):
+    1. The lines before 'go';
+    2. The lines starting with 'go' and ending with 'toolchain';
+    3. The lines after 'toolchain'.
+    """
+    parts: tuple[list[str], list[str], list[str]] = ([], [], [])
+    index = 0
+    for line in contents.splitlines(keepends=True):
+        next_index = index
+        if line.startswith('go '):
+            index = next_index = 1
+        if line.startswith('toolchain '):
+            next_index = 2
+        parts[index].append(line)
+        index = next_index
+    return parts
+
+
+def get_file_contents_from_git_revision(filename: str, revision: str) -> str:
+    """
+    Get the file contents of ``filename`` from Git revision ``revision``.
+    """
+    p = subprocess.run(
+        ['git', 'show', f'{revision}:{filename}'],
+        stdout=subprocess.PIPE,
+        check=True,
+        encoding='utf-8',
+    )
+    return p.stdout
+
+
+def read_file(filename: str) -> str:
+    """
+    Read the file's contents.
+    """
+    with open(filename, 'r', encoding='utf-8') as f:
+        return f.read()
+
+
+def write_file(filename: str, contents: str) -> None:
+    """
+    Write the file's contents.
+    """
+    with open(filename, 'w', encoding='utf-8') as f:
+        f.write(contents)
+
+
+def main():
+    """
+    Patches go.mod.
+    """
+    filename = 'go.mod'
+    _, go_versions, __ = split_go_mod(
+        get_file_contents_from_git_revision(filename, 'HEAD')
+    )
+    head, _, tail = split_go_mod(read_file(filename))
+    lines = head + go_versions + tail
+    write_file(filename, ''.join(lines))
+
+
+if __name__ == '__main__':
+    main()