build: fix -ffile-prefix-map

cmake doesn't set a `-ffile-prefix-map` for source files. Among other things,
this results in absolute paths in Scylla logs:

```
Jan 11 09:59:11.462214 longevity-tls-50gb-3d-master-db-node-2dcd4a4a-5 scylla[16339]: scylla: /jenkins/workspace/scylla-master/next/scylla/utils/refcounted.hh:23: utils::refcounted::~refcounted(): Assertion `_count == 0' failed.
```

And it results in absolute paths in gdb, which makes it a hassle to get gdb to display
source code during debugging. (A build-specific `substitute-path` has to be
configured for that).

There is a `-file-prefix-map` rule for `CMAKE_BINARY_DIR`,
but it's wrong.
Patch dbb056f4f7, which added it,
was misguided.

What we want is to strip the leading components of paths up to
the repository directory, both in __FILE__ macros and in debug info.
For example, we want to convert /home/michal/scylla/replica/table.cc to
replica/table.cc or ./replica/table.cc, both in Scylla logs and in gdb.

What the current rule does is it maps `/home/michal/scylla/build` to `.`,
which is wrong: it doesn't do anything about the paths outside of `build`,
which are the ones we actually care about.

This patch fixes the problem.

Closes scylladb/scylladb#22311

cmake/mode.common.cmake

@@ -83,7 +83,39 @@ function(get_padded_dynamic_linker_option output length)
   set(${output} "${dynamic_linker_option}=${padded_dynamic_linker}" PARENT_SCOPE)
 endfunction()
 
+# We want to strip the absolute build paths from the binary,
+# so that logs and debuggers show e.g. ./main.cc,
+# not /var/lib/jenkins/workdir/scylla/main.cc, or something.
+#
+# The base way to do that is -ffile-prefix-map=${CMAKE_SOURCE_DIR}/=
+# But by itself, it results in *both* DW_AT_name and DW_AT_comp_dir being
+# subject to the substitution.
+# For example, if table::query() is located
+# in /home/user/scylla/replica/table.cc,
+# and the compiler working directory is /home/user/scylla/build,
+# then after the ffile-prefix-map substitution it will
+# have DW_AT_comp_dir equal to ./build
+# and DW_AT_name equal to ./replica/table.cc
+#
+# If DW_AT_name is a relative path, gdb looks for the source files in $DW_AT_comp_dir/$DW_AT_name.
+# This results in e.g. gdb looking for seastar::thread_context::main
+# in ./build/./replica/table.cc
+# instead of replica/table.cc as we would like.
+# To unscrew this, we have to add a rule which will 
+# convert the /absolute/path/to/build to `.`,
+# which will result in gdb looking in ././replica/table.cc, which is fine.
+#
+# The build rule which converts `/absolute/path/to/build/` (note trailing slash)
+# to `build/` exists just so that any DW_AT_name under build (e.g. in generated sources)
+# is excluded from the first rule.
+#
+# Note that the order of these options is important.
+# Each is strictly more specific than the previous one.
+# If they were the other way around, only the most general rule would be used.
+add_compile_options("-ffile-prefix-map=${CMAKE_SOURCE_DIR}/=")
 add_compile_options("-ffile-prefix-map=${CMAKE_BINARY_DIR}=.")
+cmake_path(GET CMAKE_BINARY_DIR FILENAME build_dir_name)
+add_compile_options("-ffile-prefix-map=${CMAKE_BINARY_DIR}/=${build_dir_name}")
 
 default_target_arch(target_arch)
 if(target_arch)

configure.py

@@ -1891,6 +1891,29 @@ def configure_seastar(build_dir, mode, mode_config):
         for flag in COVERAGE_INST_FLAGS:
             seastar_cxx_ld_flags = seastar_cxx_ld_flags.replace(' ' + flag, '')
             seastar_cxx_ld_flags = seastar_cxx_ld_flags.replace(flag, '')
+    # There is a global `-ffile-prefix-map={curdir}=.` above.
+    # By itself, it results in *both* DW_AT_name and DW_AT_comp_dir being
+    # subject to the substitution.
+    # For example, if seastar::thread_context::main is located
+    # in /home/user/scylla/seastar/src/core/thread.cc,
+    # and the compiler working directory is /home/user/scylla/seastar/build/seastar,
+    # then after the ffile-prefix-map substitution it will
+    # have DW_AT_comp_dir equal to ./build/seastar
+    # and DW_AT_name equal to ./seastar/src/core/thread.cc
+    #
+    # If DW_AT_name is a relative path, gdb looks for the source files in $DW_AT_comp_dir/$DW_AT_name.
+    # This results in e.g. gdb looking for seastar::thread_context::main
+    # in ./build/seastar/./seastar/src/core/thread.cc,
+    # instead of seastar/src/core/thread.cc as we would like.
+    # To unscrew this, we have to add a rule which will 
+    # convert the /absolute/path/to/build/seastar to `.`,
+    # which will result in gdb looking in ././seastar/src/core/thread.cc, which is fine.
+    #
+    # The second build rule, which converts `/absolute/path/to/build/seastar/` (note trailing slash)
+    # to seastar/ exists just so any possible DW_AT_name under build (e.g. if there are some generated
+    # sources) is excluded from the first rule.
+    seastar_build_dir = os.path.join(build_dir, mode, 'seastar')
+    extra_file_prefix_map = f' -ffile-prefix-map={seastar_build_dir}=. -ffile-prefix-map={seastar_build_dir}/=seastar/'
     seastar_cmake_args = [
         '-DCMAKE_BUILD_TYPE={}'.format(mode_config['cmake_build_type']),
         '-DCMAKE_C_COMPILER={}'.format(args.cc),
@@ -1898,7 +1921,7 @@ def configure_seastar(build_dir, mode, mode_config):
         '-DCMAKE_EXPORT_NO_PACKAGE_REGISTRY=ON',
         '-DCMAKE_CXX_STANDARD=23',
         '-DCMAKE_CXX_EXTENSIONS=ON',
-        '-DSeastar_CXX_FLAGS=SHELL:{}'.format(mode_config['lib_cflags']),
+        '-DSeastar_CXX_FLAGS=SHELL:{}'.format(mode_config['lib_cflags'] + extra_file_prefix_map),
         '-DSeastar_LD_FLAGS={}'.format(semicolon_separated(mode_config['lib_ldflags'], seastar_cxx_ld_flags)),
         '-DSeastar_API_LEVEL=7',
         '-DSeastar_DEPRECATED_OSTREAM_FORMATTERS=OFF',
@@ -1927,7 +1950,6 @@ def configure_seastar(build_dir, mode, mode_config):
         seastar_cmake_args += ['-DBUILD_SHARED_LIBS=ON']
 
     cmake_args = seastar_cmake_args[:]
-    seastar_build_dir = os.path.join(build_dir, mode, 'seastar')
     seastar_cmd = ['cmake', '-G', 'Ninja', real_relpath(args.seastar_path, seastar_build_dir)] + cmake_args
     cmake_dir = seastar_build_dir
     if dpdk: