build: Fix possible race condition when compiling blueprint files

Due to an incompatibility between meson and our setup.

build-aux/compile-blueprints.sh

@@ -0,0 +1,68 @@
+#!/usr/bin/env bash
+#
+# Compile blueprint files to UI files all in the same directory.
+#
+# Usage: ./compile-blueprints.sh PATH_TO_BLUEPRINT_COMPILER OUTPUT_DIR BASE_INPUT_DIR [INPUT_FILE…]
+#
+# This should be a temporary solution to fix an incompatibility between our
+# setup, blueprint-compiler and meson.
+#
+# The problem is that blueprint advises to use `custom_target()` with `output`
+# set to `.`: https://gitlab.gnome.org/GNOME/blueprint-compiler/-/blob/70f32dc069c3ff13a54f24f24c0e720b938f587d/docs/setup.rst#L55
+# This doesn't work well with ninja's caching, as it doesn't know which files
+# are actually generated, so it might consider that the output is generated too
+# soon, and proceed to compiling the gresource before all blueprint files are
+# actually compiled.
+#
+# The fix for this is to actually set the proper list of generated files for the
+# output. This doesn't work with our setup because we generate files in
+# subdirectories and `custom_target`'s `output` only accepts a list of files in
+# the current directory.
+#
+# So we have 2 options to fix this:
+#
+# 1. Use a `meson.build` file per subdirectory to compile the blueprints
+#    separately which is tedious and results in a lot of duplication.
+# 2. Keep our current setup and find a way to generate a list of outputs that is
+#    accepted by meson.
+#
+# For simplicity, we want to use the second option and it would be easy if we
+# could use `generator()` to compile the blueprints. It requires
+# `gnome.compile_resources()` to accept a `generated_list` in `dependencies`,
+# but it doesn't: https://github.com/mesonbuild/meson/issues/12336.
+#
+# So this implements another solution: compile all the UI files in the same
+# directory by using a name scheme that avoids collisions. Then we can overwrite
+# the path where the resource will be available in the gresource file
+# definition by setting an `alias`.
+#
+# The output files will be located in `OUTPUT_DIR` and their names are the
+# path of `INPUT_FILE` relative to the `BASE_INPUT_DIR` with the slashes
+# replaced by `-`, and the extension changed.
+
+set -e
+
+compiler="$1"
+shift
+output_dir="$1"
+shift
+base_input_dir="$1"
+shift
+
+# For debugging.
+# echo "Compiling files in $input_dir to $output_dir with $compiler"
+
+for input_file in "$@"
+do
+    # Change extension.
+    output_file="${input_file%.blp}.ui"
+    # Remove base input dir to get relative path.
+    output_file="${output_file#$base_input_dir\/}"
+    # Replace slashes with dashes.
+    output_file="$output_dir/${output_file//\//-}"
+
+    # For debugging
+    # echo "Compiling $input_file to $output_file"
+
+    "$compiler" compile --output "$output_file" "$input_file"
+done

src/meson.build

@@ -1,43 +1,48 @@
 if not build_env_only
     # Compile Blueprint resources
     blp_files = []
-    foreach line : fs.read('ui-blueprint-resources.in').splitlines()
-        if not line.startswith('#') and line.strip() != ''
-            blp_files += line
+    ui_files = []
+    ui_files_xml = ''
+
+    # These files are already added manually in the resources file because they
+    # need custom handling.
+    blp_gresource_exceptions = [
+        'shortcuts-dialog.blp',  # Must use the root prefix for auto-detection, it must not be under `/ui`.
+    ]
+
+    foreach file : fs.read('ui-blueprint-resources.in').splitlines()
+        file = file.strip()
+
+        if file.startswith('#') or file == ''
+            continue
+        endif
+
+        blp_files += file
+
+        # We need to put all output files in the same directory
+        ui_alias = file.replace('.blp', '.ui')
+        ui_file = ui_alias.replace('/', '-')
+        ui_files += ui_file
+
+        if not blp_gresource_exceptions.contains(file)
+            ui_files_xml += '\n    <file compressed="true" preprocess="xml-stripblanks" alias="' + ui_alias + '">' + ui_file + '</file>'
         endif
     endforeach
 
     blueprints = custom_target(
         'blueprints',
         input: blp_files,
-        output: '.',
+        output: ui_files,
         command: [
-            blueprint_compiler,
-            'batch-compile',
-            '@OUTPUT@',
+            '../build-aux/compile-blueprints.sh',
+            blueprint_compiler.full_path(),
+            '@OUTDIR@',
             '@CURRENT_SOURCE_DIR@',
             '@INPUT@',
         ],
     )
 
-    # Populate the GResource file for the compiled Blueprint files dynamically,
-    # using the same file list.
-    # These files are already added manually in the resources file because they
-    # need custom handling.
-    blp_exceptions = [
-        'shortcuts-dialog.blp',  # Must use the root prefix for auto-detection, it must not be under `/ui`.
-    ]
-
-    ui_files_xml = ''
-    foreach file : blp_files
-        if not blp_exceptions.contains(file)
-            ui_files_xml += '\n    <file compressed="true" preprocess="xml-stripblanks">' + file.replace(
-                '.blp',
-                '.ui',
-            ) + '</file>'
-        endif
-    endforeach
-
+    # Populate the GResource file for the compiled Blueprint files dynamically.
     ui_resources_xml = configure_file(
         input: 'ui-resources.gresource.xml.in',
         output: 'ui-resources.gresource.xml',