systemd: selectively backport "Fix constness issues with newer glibc"

NetworkManager is failing to build on Rawhide with the following errors:
../src/libnm-systemd-shared/src/basic/string-util.h:33:16: error: return discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
   33 |         return strstr(haystack, needle);
      |                ^~~~~~
In file included from ../src/libnm-systemd-shared/src/basic/fd-util.c:30:
../src/libnm-systemd-shared/src/basic/sort-util.h: In function ‘bsearch_safe’:
../src/libnm-systemd-shared/src/basic/sort-util.h:34:16: error: return discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
   34 |         return bsearch(key, base, nmemb, size, compar);
      |                ^~~~~~~

This is fixed in systemd by commit 0bac1ed2422f15308414dd1e9d09812a966b0348:
> Latest glibc uses _Generic to have strstr() and other functions return
> const char* or char* based on whether the input is a const char* or a
> char*. This causes build failures as we previously always expected a char*.
>
> Let's fix the compilation failures and add our own macros similar to glibc's
> to have string functions that return a mutable or const pointer depending on
> the input.

Selectively backport the changes we need to fix building.

src/libnm-systemd-shared/src/basic/sort-util.h

@@ -25,15 +25,18 @@ void *xbsearch_r(const void *key, const void *base, size_t nmemb, size_t size,
  * Normal bsearch requires base to be nonnull. Here were require
  * that only if nmemb > 0.
  */
-static inline void* bsearch_safe(const void *key, const void *base,
+static inline void* bsearch_safe_internal(const void *key, const void *base,
                                  size_t nmemb, size_t size, comparison_fn_t compar) {
         if (nmemb <= 0)
                 return NULL;
 
         assert(base);
-        return bsearch(key, base, nmemb, size, compar);
+        return (void*) bsearch(key, base, nmemb, size, compar);
 }
 
+#define bsearch_safe(key, base, nmemb, size, compar) \
+        const_generic((base), bsearch_safe_internal(key, base, nmemb, size, compar))
+
 #define typesafe_bsearch(k, b, n, func)                                 \
         ({                                                              \
                 const typeof((b)[0]) *_k = k;                           \

src/libnm-systemd-shared/src/basic/string-util.c

@@ -1470,7 +1470,7 @@ ssize_t strlevenshtein(const char *x, const char *y) {
         return t1[yl];
 }
 
-char* strrstr(const char *haystack, const char *needle) {
+char* strrstr_internal(const char *haystack, const char *needle) {
         /* Like strstr() but returns the last rather than the first occurrence of "needle" in "haystack". */
 
         if (!haystack || !needle)
@@ -1479,7 +1479,7 @@ char* strrstr(const char *haystack, const char *needle) {
         /* Special case: for the empty string we return the very last possible occurrence, i.e. *after* the
          * last char, not before. */
         if (*needle == 0)
-                return strchr(haystack, 0);
+                return (char*) strchr(haystack, 0);
 
         for (const char *p = strstr(haystack, needle), *q; p; p = q) {
                 q = strstr(p + 1, needle);

src/libnm-systemd-shared/src/basic/string-util.h

@@ -27,24 +27,28 @@
 #define URI_UNRESERVED      ALPHANUMERICAL "-._~"       /* [RFC3986] */
 #define URI_VALID           URI_RESERVED URI_UNRESERVED /* [RFC3986] */
 
-static inline char* strstr_ptr(const char *haystack, const char *needle) {
+static inline char* strstr_ptr_internal(const char *haystack, const char *needle) {
         if (!haystack || !needle)
                 return NULL;
-        return strstr(haystack, needle);
+        return (char*) strstr(haystack, needle);
 }
 
-static inline char* strstrafter(const char *haystack, const char *needle) {
-        char *p;
+#define strstr_ptr(haystack, needle) \
+        const_generic(haystack, strstr_ptr_internal(haystack, needle))
 
+static inline char* strstrafter_internal(const char *haystack, const char *needle) {
         /* Returns NULL if not found, or pointer to first character after needle if found */
 
-        p = strstr_ptr(haystack, needle);
+        char *p = (char*) strstr_ptr(haystack, needle);
         if (!p)
                 return NULL;
 
         return p + strlen(needle);
 }
 
+#define strstrafter(haystack, needle) \
+        const_generic(haystack, strstrafter_internal(haystack, needle))
+
 static inline const char* strnull(const char *s) {
         return s ?: "(null)";
 }
@@ -300,6 +304,8 @@ bool version_is_valid_versionspec(const char *s);
 
 ssize_t strlevenshtein(const char *x, const char *y);
 
-char* strrstr(const char *haystack, const char *needle);
+char* strrstr_internal(const char *haystack, const char *needle);
+#define strrstr(haystack, needle) \
+        const_generic(haystack, strrstr_internal(haystack, needle))
 
 size_t str_common_prefix(const char *a, const char *b);

src/libnm-systemd-shared/src/fundamental/macro-fundamental.h

@@ -511,4 +511,11 @@ assert_cc(STRLEN(__FILE__) > STRLEN(RELATIVE_SOURCE_PATH) + 1);
 #define PROJECT_FILE (&__FILE__[STRLEN(RELATIVE_SOURCE_PATH) + 1])
 #else /* NM_IGNORED */
 #define PROJECT_FILE __FILE__
-#endif /* NM_IGNORED */
+#endif /* NM_IGNORED */
+
+/* This macro is used to have a const-returning and non-const returning version of a function based on
+ * whether its first argument is const or not (e.g. strstr()). */
+#define const_generic(ptr, call)                              \
+        _Generic(0 ? (ptr) : (void*) 1,                       \
+                 const void*: (const typeof(*call)*) (call),  \
+                 void*: (call))