nestlevel: Fix user data alignment

We need to align the user data properly not to trigger undefined
behavior, which even apparently crashes on SPARC.

As `NestingLevels::levels` is actually a single allocation for all
levels and their user data mapped as `[NL0|UD0|NL1|UD1|...]` (where NL
is a NestingLevel, and UD a user data), we need to align twice, as we
need every `NL*` and every `UD*` to align properly.

Here we align everything to `2*sizeof(size_t)`, which is a logic
borrowed from GLib, which seems to have borrowed the value from glibc.
This is pretty conservative in our case, because actually `NL*`s only
need aligning to `int`'s requirements currently, which on some
architectures is 4, not 16; but it's trickier to implement (and
actually impossible with the current API) as we'd need to compute the
actual alignment for each level taking into account it's position in
the overall memory region to still align `UD*`s to a conservative
value.
Also, having all NL+UD group at the same size makes things a bit
simpler for debugging, I guess.

We make sure to only add alignment padding manually for cases where
there's actually some user data, not to waste memory needlessly for the
common case where `sizeof(UD)` is 0, and thus where we can merely
align to `sizeof(NL)` -- which C does for us already.

Note that currently only the Ruby parser is affected, as it's the only
current consumer of nesting level user data.

Fixes #3881.

Author: b4n

main/nestlevel.c

@@ -20,8 +20,16 @@
 
 #include <string.h>
 
-/* TODO: Alignment */
-#define NL_SIZE(nls) (sizeof(NestingLevel) + (nls)->userDataSize)
+/* struct alignment trick, copied from GObject's gtype.c, which borrows
+ * 2*szieof(size_t) from glibc */
+#define STRUCT_ALIGNMENT (2 * sizeof (size_t))
+#define ALIGN_STRUCT(offset) ((offset + (STRUCT_ALIGNMENT - 1)) & -STRUCT_ALIGNMENT)
+
+/* account for the user data alignment if we have user data, otherwise allocate
+ * exactly what's needed not to waste memory for unneeded alignment */
+#define NL_SIZE(nls) ((nls)->userDataSize ? (ALIGN_STRUCT (sizeof (NestingLevel)) + ALIGN_STRUCT ((nls)->userDataSize)) : sizeof (NestingLevel))
+#define NL_USER_DATA(nl) ((void *)(((char *) nl) + ALIGN_STRUCT (sizeof (NestingLevel))))
+
 #define NL_NTH(nls,n) (NestingLevel *)(((char *)((nls)->levels)) + ((n) * NL_SIZE (nls)))
 
 /*
@@ -73,7 +81,7 @@ extern NestingLevel * nestingLevelsPush(NestingLevels *nls, int corkIndex)
 
 	nl->corkIndex = corkIndex;
 	if (nls->userDataSize > 0)
-		memset (nl->userData, 0, nls->userDataSize);
+		memset (NL_USER_DATA (nl), 0, ALIGN_STRUCT (nls->userDataSize));
 
 	return nl;
 }
@@ -117,5 +125,5 @@ extern NestingLevel *nestingLevelsGetNthParent (const NestingLevels *nls, int n)
 
 extern void *nestingLevelGetUserData (const NestingLevel *nl)
 {
-	return (void *)nl->userData;
+	return NL_USER_DATA (nl);
 }

main/nestlevel.h

@@ -26,7 +26,8 @@ typedef struct NestingLevels NestingLevels;
 struct NestingLevel
 {
 	int corkIndex;
-	char userData [];
+	/* user data is allocated at the end of this struct (possibly with some
+	 * offset for alignment), get it with nestingLevelGetUserData() */
 };
 
 struct NestingLevels