hwdb: fix dangling 'else' ambuguity

Imagine the following use of hwdb:

    if (condition_A)
        SD_HWDB_FOREACH_PROPERTY(hwdb, modalias, key, value)
            operation_A(key, value);
    else
        log_error("...");

This should work just fine, but but definitely does not what you would
expect. Due to how SD_HWDB_FOREACH_PROPERTY is defined, the dangling
'else' is linked to the hidden 'if' statement in the macro instead of the
outer 'if (condition_A)'. This is unexpected and really annoying to debug.

Fix this by never leaving un-finished if-statements in
SD_HWDB_FOREACH_PROPERTY(). We simply inverse the if() statement and
explicitly add an 'else'-branch. This way, the statement is closed and all
ambuguities are resolved.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/systemd/sd-hwdb.h b/src/systemd/sd-hwdb.h
index f41555df7e..3c44b981d6 100644
--- a/src/systemd/sd-hwdb.h
+++ b/src/systemd/sd-hwdb.h
@@ -39,8 +39,9 @@ int sd_hwdb_get(sd_hwdb *hwdb, const char *modalias, const char *key, const char
 int sd_hwdb_seek(sd_hwdb *hwdb, const char *modalias);
 int sd_hwdb_enumerate(sd_hwdb *hwdb, const char **key, const char **value);
 
+/* the inverse condition avoids ambiguity of danling 'else' after the macro */
 #define SD_HWDB_FOREACH_PROPERTY(hwdb, modalias, key, value)            \
-        if (sd_hwdb_seek(hwdb, modalias) >= 0)                          \
-                while (sd_hwdb_enumerate(hwdb, &(key), &(value)) > 0)
+        if (sd_hwdb_seek(hwdb, modalias) < 0) { }                       \
+        else while (sd_hwdb_enumerate(hwdb, &(key), &(value)) > 0)
 
 #endif