Use const char pointer in redismodule.h as far as possible (#10064)

When I used C++ to develop a redis module. i  used `string.data()` as the second parameter `ele`
of  `RedisModule_DigestAddStringBuffer`, but there is a warning, since we never change the `ele`,
i think we should use `const char` for it.

This PR adds const to just a handful of module APIs that required it, all not very widely used.
The implication is a breaking change in terms of compilation error that's easy to resolve, and no ABI impact.
The affected APIs are around Digest, Info injection, and Cluster bus messages.

1 files changed, 5 insertions, 6 deletions

diff --git a/src/debug.c b/src/debug.c
index c8a6fc186..2da2c5d50 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -79,13 +79,13 @@ void logStackTrace(void *eip, int uplevel);
  * "add" digests relative to unordered elements.
  *
  * So digest(a,b,c,d) will be the same of digest(b,a,c,d) */
-void xorDigest(unsigned char *digest, void *ptr, size_t len) {
+void xorDigest(unsigned char *digest, const void *ptr, size_t len) {
     SHA1_CTX ctx;
-    unsigned char hash[20], *s = ptr;
+    unsigned char hash[20];
     int j;
 
     SHA1Init(&ctx);
-    SHA1Update(&ctx,s,len);
+    SHA1Update(&ctx,ptr,len);
     SHA1Final(hash,&ctx);
 
     for (j = 0; j < 20; j++)
@@ -112,11 +112,10 @@ void xorStringObjectDigest(unsigned char *digest, robj *o) {
  * Also note that mixdigest("foo") followed by mixdigest("bar")
  * will lead to a different digest compared to "fo", "obar".
  */
-void mixDigest(unsigned char *digest, void *ptr, size_t len) {
+void mixDigest(unsigned char *digest, const void *ptr, size_t len) {
     SHA1_CTX ctx;
-    char *s = ptr;
 
-    xorDigest(digest,s,len);
+    xorDigest(digest,ptr,len);
     SHA1Init(&ctx);
     SHA1Update(&ctx,digest,20);
     SHA1Final(digest,&ctx);