From f44c15b7d501f0f4ce6717e82b3bbf45051c3589 Mon Sep 17 00:00:00 2001
From: Alex Hochheiden <ahochheiden@mozilla.com>
Date: Tue, 24 Mar 2026 01:49:21 +0000
Subject: [PATCH] Bug 2023319 - Error on Windows when `topsrcdir` or
 `topobjdir` paths are too long r=firefox-build-system-reviewers,nalexander

We can't share code with `bin\bootstrap.py`, so there's a minor amount of
duplication, but it's better to notify the user before they clone than
during the first `./mach configure` run.

Differential Revision: https://phabricator.services.mozilla.com/D289380
---
 configure.py                    | 39 +++++++++++++++++++++++++++++++++
 python/mozboot/bin/bootstrap.py | 20 +++++++++++++++++
 2 files changed, 59 insertions(+)

diff --git a/configure.py b/configure.py
index c291e7123049..b8b217f4891b 100644
--- a/configure.py
+++ b/configure.py
@@ -100,6 +100,45 @@ def main(argv):
             )
             return 1
 
+        if sys.platform == "win32":
+            # Long paths cause two kinds of build failures on Windows:
+            # 1. Tools like midl.exe do not support paths exceeding MAX_PATH
+            #    (260 characters), even with the LongPathsEnabled registry setting.
+            # 2. Long source paths repeated across many -I flags can exceed
+            #    the CreateProcessW command line limit of 32,767 characters.
+            WIN32_MAX_PATH = 260
+            LONGEST_KNOWN_OBJDIR_RELATIVE_PATH = 170
+            DEFAULT_OBJDIR_NAME_LEN = 28  # /obj-x86_64-pc-windows-msvc/
+            max_objdir_len = WIN32_MAX_PATH - LONGEST_KNOWN_OBJDIR_RELATIVE_PATH
+            # Account for the default objdir name in the srcdir limit so that
+            # a user doesn't fix a srcdir error only to immediately hit the
+            # objdir error with the default configuration.
+            max_srcdir_len = max_objdir_len - DEFAULT_OBJDIR_NAME_LEN
+            if len(topsrcdir) > max_srcdir_len:
+                print(
+                    f"Source directory path ({topsrcdir}) is "
+                    f"{len(topsrcdir)} characters, which exceeds the "
+                    f"Windows limit of {max_srcdir_len}.\n"
+                    f"Move your source directory to a shorter path "
+                    f"(e.g. D:\\mozilla-source\\firefox).",
+                    file=sys.stderr,
+                )
+                return 1
+            if len(topobjdir) > max_objdir_len:
+                print(
+                    f"Object directory path ({topobjdir}) is "
+                    f"{len(topobjdir)} characters, which exceeds the "
+                    f"Windows limit of {max_objdir_len}.\n"
+                    + (
+                        "Move your source directory to a shorter path or set "
+                        "MOZ_OBJDIR to a shorter absolute path in your mozconfig."
+                        if topobjdir.startswith(topsrcdir)
+                        else "Set MOZ_OBJDIR to a shorter absolute path in your mozconfig."
+                    ),
+                    file=sys.stderr,
+                )
+                return 1
+
         # Do not allow topobjdir == topsrcdir
         if os.path.samefile(topsrcdir, topobjdir):
             print(
diff --git a/python/mozboot/bin/bootstrap.py b/python/mozboot/bin/bootstrap.py
index 6be38685493e..12ceabc38dd9 100755
--- a/python/mozboot/bin/bootstrap.py
+++ b/python/mozboot/bin/bootstrap.py
@@ -71,6 +71,26 @@ def which(name):
 def validate_clone_dest(dest: Path):
     dest = dest.resolve()
 
+    if WINDOWS:
+        # Keep in sync with the path length checks in configure.py.
+        WIN32_MAX_PATH = 260
+        LONGEST_KNOWN_OBJDIR_RELATIVE_PATH = 170
+        DEFAULT_OBJDIR_NAME_LEN = 28  # /obj-x86_64-pc-windows-msvc/
+        max_srcdir_len = (
+            WIN32_MAX_PATH
+            - LONGEST_KNOWN_OBJDIR_RELATIVE_PATH
+            - DEFAULT_OBJDIR_NAME_LEN
+        )
+        dest_len = len(str(dest))
+        if dest_len > max_srcdir_len:
+            print(
+                f"ERROR! Destination path ({dest}) is {dest_len} characters, "
+                f"which exceeds the Windows limit of {max_srcdir_len}. "
+                f"This will cause build failures due to path length restrictions.\n"
+                f"Please choose a shorter path (e.g. D:\\mozilla-source\\firefox)."
+            )
+            return None
+
     if not dest.exists():
         return dest