waf: apply coverage flags after the configure checks have run

The coverage flags were added in configure_env, so every probe program
built by cfg.check() was compiled with -fprofile-arcs and linked with
-lgcov.  On boards which also link with -Wl,--wrap,malloc (SITL, Linux,
QURT) libgcov's malloc call is rewritten to __wrap_malloc, which the
probe does not define, so the probe fails to link and the feature under
test is silently recorded as absent.  Under --coverage every header
check ("endian.h", "byteswap.h", the cmath checks, memrchr) reported
"not found"; HAVE_BYTESWAP_H was never defined and the fallback in
AP_Common/missing/byteswap.h was compiled instead.  That fallback
collides with glibc's own __bswap_16/32/64 as soon as a translation
unit also includes netinet/in.h, which broke build.unit_tests in the
weekly coverage workflow.

Move the flags into Board.configure_coverage() and call it from the
top-level configure once the checks are done, so the probes are never
instrumented and the coverage configuration produces the same
ap_config.h as a normal one.

Co-authored-by: Pierre Kancir <pierre.kancir.emn@gmail.com>

Tools/ardupilotwaf/boards.py

@@ -248,6 +248,24 @@ class Board:
         return (int(major) < want_major or
                 (int(major) == want_major and int(minor) <= want_minor))
 
+    def configure_coverage(self, cfg):
+        """Apply the coverage flags.
+
+        Called after the configure checks have run, rather than from
+        configure_env: cfg.check() links small probe programs, and
+        instrumenting those drags in libgcov, whose malloc call
+        -Wl,--wrap,malloc rewrites into an undefined __wrap_malloc.  The
+        probe then fails to link and waf records the feature it was
+        probing for as missing.
+        """
+        if not cfg.env.COVERAGE:
+            return
+        cfg.env.CFLAGS += ['-fprofile-arcs', '-ftest-coverage']
+        cfg.env.CXXFLAGS += ['-fprofile-arcs', '-ftest-coverage']
+        cfg.env.LINKFLAGS += ['-lgcov', '-coverage']
+        # cfg.env is post-merge, where DEFINES is a list of NAME=value
+        cfg.env.DEFINES += ['HAL_COVERAGE_BUILD=1']
+
     def configure_env(self, cfg, env):
         # Use a dictionary instead of the conventional list for definitions to
         # make easy to override them. Convert back to list before consumption.
@@ -342,23 +360,6 @@ class Board:
             env.CFLAGS += [
                 '-g',
             ]
-        if cfg.env.COVERAGE:
-            env.CFLAGS += [
-                '-fprofile-arcs',
-                '-ftest-coverage',
-            ]
-            env.CXXFLAGS += [
-                '-fprofile-arcs',
-                '-ftest-coverage',
-            ]
-            env.LINKFLAGS += [
-                '-lgcov',
-                '-coverage',
-            ]
-            env.DEFINES.update(
-                HAL_COVERAGE_BUILD = 1,
-            )
-
         if cfg.options.bootloader:
             # don't let bootloaders try and pull scripting in
             cfg.options.disable_scripting = True

wscript

@@ -628,6 +628,7 @@ def configure(cfg):
         else:
             cfg.end_msg('disabled', color='YELLOW')
 
+    cfg.get_board().configure_coverage(cfg)
     cfg.start_msg('Coverage build')
     if cfg.env.COVERAGE:
         cfg.end_msg('enabled')