mtest: only build what is needed for the tests

It is a usual workflow to fix something and retest to see if it is fixed using a
particular test.  When tests start to become numerous, it becomes time consuming
for "meson test" to relink all of them (and in fact rebuild the whole project)
where the user has already specified the tests they want to run, as well as
the tests' dependencies.

Teach meson to be smart and only build what is needed for the test (or suite)
that were specified.

Fixes: #7473
Related: #7830

docs/markdown/snippets/meson_test_depends.md

@@ -0,0 +1,17 @@
+## `meson test` only rebuilds test dependencies
+
+Until now, `meson test` rebuilt the whole project independent of the
+requested tests and their dependencies.  With this release, `meson test`
+will only rebuild what is needed for the tests or suites that will be run.
+This feature can be used, for example, to speed up bisecting regressions
+using commands like the following:
+
+    git bisect start <broken commit> <working commit>
+    git bisect run meson test <failing test name>
+
+This would find the broken commit automatically while at each step
+rebuilding only those pieces of code needed to run the test.
+
+However, this change could cause failures if dependencies are not
+specified correctly in `meson.build`.
+

mesonbuild/minstall.py

@@ -19,7 +19,6 @@ from glob import glob
 from .scripts import depfixer
 from .scripts import destdir_join
 from .mesonlib import is_windows, Popen_safe
-from .mtest import rebuild_all
 from .backend.backends import InstallData
 from .coredata import major_versions_differ, MesonVersionMismatchException
 from .coredata import version as coredata_version
@@ -532,6 +531,23 @@ class Installer:
                     else:
                         raise
 
+def rebuild_all(wd: str) -> bool:
+    if not (Path(wd) / 'build.ninja').is_file():
+        print('Only ninja backend is supported to rebuild the project before installation.')
+        return True
+
+    ninja = environment.detect_ninja()
+    if not ninja:
+        print("Can't find ninja, can't rebuild test.")
+        return False
+
+    ret = subprocess.run(ninja + ['-C', wd]).returncode
+    if ret != 0:
+        print('Could not rebuild {}'.format(wd))
+        return False
+
+    return True
+
 def run(opts):
     datafilename = 'meson-private/install.dat'
     private_dir = os.path.dirname(datafilename)

mesonbuild/mtest.py

@@ -45,6 +45,7 @@ from .coredata import major_versions_differ, MesonVersionMismatchException
 from .coredata import version as coredata_version
 from .dependencies import ExternalProgram
 from .mesonlib import MesonException, get_wine_shortpath, split_args, join_args
+from .mintro import get_infodir, load_info_file
 from .backend.backends import TestProtocol, TestSerialisation
 
 # GNU autotools interprets a return code of 77 from tests it executes to
@@ -1019,13 +1020,21 @@ class TestHarness:
     def total_failure_count(self) -> int:
         return self.fail_count + self.unexpectedpass_count + self.timeout_count
 
-    def doit(self) -> int:
+    def doit(self, options: argparse.Namespace) -> int:
         if self.is_run:
             raise RuntimeError('Test harness object can only be used once.')
         self.is_run = True
         tests = self.get_tests()
         if not tests:
             return 0
+        if not options.no_rebuild and not rebuild_deps(options.wd, tests):
+            # We return 125 here in case the build failed.
+            # The reason is that exit code 125 tells `git bisect run` that the current
+            # commit should be skipped.  Thus users can directly use `meson test` to
+            # bisect without needing to handle the does-not-build case separately in a
+            # wrapper script.
+            sys.exit(125)
+
         self.run_tests(tests)
         return self.total_failure_count()
 
@@ -1272,7 +1281,7 @@ def list_tests(th: TestHarness) -> bool:
         print(th.get_pretty_suite(t))
     return not tests
 
-def rebuild_all(wd: str) -> bool:
+def rebuild_deps(wd: str, tests: T.List[TestSerialisation]) -> bool:
     if not (Path(wd) / 'build.ninja').is_file():
         print('Only ninja backend is supported to rebuild tests before running them.')
         return True
@@ -1282,7 +1291,21 @@ def rebuild_all(wd: str) -> bool:
         print("Can't find ninja, can't rebuild test.")
         return False
 
-    ret = subprocess.run(ninja + ['-C', wd]).returncode
+    depends = set()            # type: T.Set[str]
+    targets = set()            # type: T.Set[str]
+    intro_targets = dict()     # type: T.Dict[str, T.List[str]]
+    for target in load_info_file(get_infodir(wd), kind='targets'):
+        intro_targets[target['id']] = [
+                os.path.relpath(f, wd)
+                for f in target['filename']]
+    for t in tests:
+        for d in t.depends:
+            if d in depends:
+                continue
+            depends.update(d)
+            targets.update(intro_targets[d])
+
+    ret = subprocess.run(ninja + ['-C', wd] + sorted(targets)).returncode
     if ret != 0:
         print('Could not rebuild {}'.format(wd))
         return False
@@ -1318,18 +1341,11 @@ def run(options: argparse.Namespace) -> int:
             print('Could not find requested program: {!r}'.format(check_bin))
             return 1
 
-    if not options.list and not options.no_rebuild:
-        if not rebuild_all(options.wd):
-            # We return 125 here in case the build failed.
-            # The reason is that exit code 125 tells `git bisect run` that the current commit should be skipped.
-            # Thus users can directly use `meson test` to bisect without needing to handle the does-not-build case separately in a wrapper script.
-            return 125
-
     with TestHarness(options) as th:
         try:
             if options.list:
                 return list_tests(th)
-            return th.doit()
+            return th.doit(options)
         except TestException as e:
             print('Meson test encountered an error:\n')
             if os.environ.get('MESON_FORCE_BACKTRACE'):