fix(generate_klayout_tech): write absolute LEF paths — klayout sandbox

The previous switch to relpath (commit 01652ca) cleaned up the
content but did not actually fix def2stream's errno=2 under a Bazel
sandbox.  Investigation via strace shows klayout's
Layout.read(def, layout_options) opens the input DEF (a sandbox
symlink to the bare execroot, staged from orfs_final), realpath's
it to the bare-execroot results dir, and then resolves <lef-files>
relative paths against THAT dir.  Sibling intermediates like
objects/klayout_tech.lef only exist in the per-action sandbox, not
at the bare execroot, so the lookup lands at
.../execroot/_main/bazel-out/.../klayout_tech.lef and fails with
errno=2.

Declaring klayout_tech.lef as a Bazel output of orfs_gds doesn't
help -- Bazel only materialises declared outputs at the bare
execroot at action completion, not while the action is still
running and klayout is reading them.

Plain abspath (NOT realpath -- realpath would chase Bazel
input-file symlinks back out to the bare execroot for any input
already at the bare execroot) writes the in-sandbox absolute LEF
path into the LYT.  KLayout opens that absolute path directly with
no relative-path resolution involved, so the sibling lookup never
escapes the sandbox.

Cost: the LYT's content now embeds the per-invocation sandbox
number, so the orfs_gds action cannot be cross-action cached.
That's an acceptable trade-off for correctness while we wait for
a klayout-level fix or a Bazel feature that lets us write a
stable bare-execroot path from inside the action.

Confirmed end-to-end:
  bazelisk build //flow/designs/sky130hd/gcd:gcd_final_blender_html
produces 6_final.gds and the blender_html sh_binary, with both
the sandbox sandbox-id N going forward and after fresh action
re-runs (verified by deleting outputs and rebuilding).

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

flow/util/generate_klayout_tech.py

@@ -51,11 +51,17 @@ def generate_klayout_tech(
     with open(template_lyt, "r") as f:
         content = f.read()
 
-    # Compute relpath without realpath(): under a Bazel sandbox, bazel-out/
-    # entries are symlinks to the bare execroot; realpath follows them and
-    # makes the generated LYT reference files at the bare-execroot path,
-    # where in-flight outputs do not exist during action execution.
-    resolved_lefs = [os.path.relpath(f, reference_dir) for f in lef_files]
+    # Write absolute (not relative, not realpath'd) LEF paths into the LYT.
+    # Klayout's Layout.read(def, layout_options) follows the symlinked input
+    # DEF to its realpath at the bare execroot and resolves relative
+    # <lef-files> entries from there.  Sibling intermediates like
+    # objects/klayout_tech.lef don't exist at the bare execroot during
+    # action execution -- they're only at the per-action sandbox -- so
+    # resolution fails with errno=2.  Plain abspath (NOT realpath, which
+    # would chase Bazel input-file symlinks back out to the bare execroot)
+    # keeps klayout pointed at the in-sandbox file.  reference_dir is now
+    # unused for LEFs.
+    resolved_lefs = [os.path.abspath(f) for f in lef_files]
 
     content = replace_lef_files(content, resolved_lefs)