sphinx: kernel_abi: fix performance regression with O=<dir>

The logic there which adds a dependency note to Sphinx cache
is not taking into account that the build dir may not be
the source dir. This causes a performance regression:

$ time make O=/tmp/foo SPHINXDIRS=admin-guide htmldocs

	[OUTDATED]
	Added: set()
	Changed: {'abi-obsolete', 'abi-removed', 'abi-stable-files', 'abi-obsolete-files', 'abi-stable', 'abi', 'abi-removed-files', 'abi-testing-files', 'abi-testing', 'gpio/index', 'gpio/obsolete'}
	Removed: set()
	All docs count: 385
	Found docs count: 385

	real    0m11,324s
	user    0m15,783s
	sys     0m1,164s

To get the root cause of the problem (ABI files reported as changed),
I used this changeset:

	diff --git a/Documentation/conf.py b/Documentation/conf.py
	index e8766e689c1b..ab486623bd8b 100644
	--- a/Documentation/conf.py
	+++ b/Documentation/conf.py
	@@ -571,3 +571,16 @@ def setup(app):
	     """Patterns need to be updated at init time on older Sphinx versions"""

	     app.connect('config-inited', update_patterns)
	+    app.connect('env-get-outdated', on_outdated)
	+
	+def on_outdated(app, env, added, changed, removed):
	+    """Track cache outdated due to added/changed/removed files"""
	+    print("\n[OUTDATED]")
	+    print(f"Added: {added}")
	+    print(f"Changed: {changed}")
	+    print(f"Removed: {removed}")
	+    print(f"All docs count: {len(env.all_docs)}")
	+    print(f"Found docs count: {len(env.found_docs)}")
	+
	+    # Just return what we have
	+    return added | changed | removed

Reported-by: Akira Yokosawa <[email protected]>
Closes: https://lore.kernel.org/linux-doc/[email protected]/
Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Tested-by: Akira Yokosawa <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/e25673d87357457bc54ee863e97ff8f75956580d.1752752211.git.mchehab+huawei@kernel.org

Author: mchehab

Documentation/sphinx/kernel_abi.py

@@ -146,8 +146,10 @@ def run(self):
                 n += 1
 
             if f != old_f:
-                # Add the file to Sphinx build dependencies
-                env.note_dependency(os.path.abspath(f))
+                # Add the file to Sphinx build dependencies if the file exists
+                fname = os.path.join(srctree, f)
+                if os.path.isfile(fname):
+                    env.note_dependency(fname)
 
                 old_f = f