build: merge scylla's and seastar's compilation database

Since commit 415c83fa, Seastar is built as an external project. As a result,
the compile_commands.json file generated by ScyllaDB's CMake build system no
longer contains compilation rules for Seastar's object files. This limitation
prevents tools from performing static analysis using the complete dependency
tree of translation units.

This change merges Seastar's compilation database with ScyllaDB's and places
the combined database in the source root directory, maintaining backward
compatibility.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb/scylladb#21234

configure.py

@@ -2495,6 +2495,33 @@ class BuildType(NamedTuple):
     cmake_build_type: str
 
 
+def generate_compdb_for_cmake_build(source_dir, build_dir):
+    # Since Seastar and Scylla are configured as separate projects, their compilation
+    # databases need to be merged into a single database for tooling consumption.
+    compdb = 'compile_commands.json'
+    scylla_compdb_path = os.path.join(build_dir, compdb)
+    seastar_compdb_path = ''
+    # sort build types by supposed indexing speed
+    for build_type in ['Dev', 'Debug', 'RelWithDebInfo', 'Sanitize']:
+        seastar_compdb_path = os.path.join(build_dir, build_type, 'seastar', compdb)
+        if os.path.exists(seastar_compdb_path):
+            break
+    assert seastar_compdb_path, "Seasetar's building system is not configured yet."
+    # if the file exists, just overwrite it so we can keep it updated
+    with open(os.path.join(source_dir, compdb), 'w+b') as merged_compdb:
+        # "merge-compdb.py" considers all object files under the "--prefix"
+        # directory as relevant. Since CMake generates .o files in
+        # "CMakeFiles" directories, we preserve the compilation rules for
+        # these generated files.
+        prefix = "CMakeFiles"
+        subprocess.run([os.path.join(source_dir, 'scripts/merge-compdb.py'),
+                        prefix,
+                        scylla_compdb_path,
+                        seastar_compdb_path],
+                       stdout=merged_compdb,
+                       check=True)
+
+
 def configure_using_cmake(args):
     # all supported build modes, and if they are built by default if selected
     build_modes = {'debug': BuildType(True, 'Debug'),
@@ -2541,16 +2568,7 @@ def configure_using_cmake(args):
                       '-B', build_dir,
                       '-S', source_dir]
     subprocess.check_call(cmake_command, shell=False, cwd=source_dir)
-
-    compdb_source = os.path.join(source_dir, 'compile_commands.json')
-    compdb_target = os.path.join(build_dir, 'compile_commands.json')
-    
-    try:
-        os.symlink(compdb_target, compdb_source)
-    except FileExistsError:
-        # if there is already a valid compile_commands.json link in the
-        # source root, we are done.
-        pass
+    generate_compdb_for_cmake_build(source_dir, build_dir)
 
 
 if __name__ == '__main__':