Use a switch-statement when checking for compression method.

This patch changes the way `tor_compress_new()`,
`tor_compress_process()`, and `tor_compress_free()` handles different
compression methods. This should give us compiler warnings in case an
additional compression method is added, but the developer forgets to add
handlers in the three aforementioned functions.

See https://bugs.torproject.org/21663

src/common/compress.c

@@ -149,14 +149,22 @@ tor_compress_new(int compress, compress_method_t method,
   state = tor_malloc_zero(sizeof(tor_compress_state_t));
   state->method = method;
 
-  if (method == GZIP_METHOD || method == ZLIB_METHOD) {
-    tor_zlib_compress_state_t *zlib_state =
-      tor_zlib_compress_new(compress, method, compression_level);
+  switch (method) {
+    case GZIP_METHOD:
+    case ZLIB_METHOD: {
+      tor_zlib_compress_state_t *zlib_state =
+        tor_zlib_compress_new(compress, method, compression_level);
 
-    if (zlib_state == NULL)
-      goto err;
+      if (zlib_state == NULL)
+        goto err;
+
+      state->u.zlib_state = zlib_state;
+      break;
+    }
 
-    state->u.zlib_state = zlib_state;
+    case NO_METHOD:
+    case UNKNOWN_METHOD:
+      goto err;
   }
 
   return state;
@@ -185,11 +193,19 @@ tor_compress_process(tor_compress_state_t *state,
 {
   tor_assert(state != NULL);
 
-  if (state->method == GZIP_METHOD || state->method == ZLIB_METHOD)
-    return tor_zlib_compress_process(state->u.zlib_state,
-                                     out, out_len, in, in_len,
-                                     finish);
+  switch (state->method) {
+    case GZIP_METHOD:
+    case ZLIB_METHOD:
+      return tor_zlib_compress_process(state->u.zlib_state,
+                                       out, out_len, in, in_len,
+                                       finish);
 
+    case NO_METHOD:
+    case UNKNOWN_METHOD:
+      goto err;
+  }
+
+ err:
   return TOR_COMPRESS_ERROR;
 }
 
@@ -200,8 +216,16 @@ tor_compress_free(tor_compress_state_t *state)
   if (state == NULL)
     return;
 
-  if (state->method == GZIP_METHOD || state->method == ZLIB_METHOD)
-    tor_zlib_compress_free(state->u.zlib_state);
+  switch (state->method) {
+    case GZIP_METHOD:
+    case ZLIB_METHOD:
+      tor_zlib_compress_free(state->u.zlib_state);
+      break;
+
+    case NO_METHOD:
+    case UNKNOWN_METHOD:
+      break;
+  }
 
   tor_free(state);
 }
@@ -212,9 +236,17 @@ tor_compress_state_size(const tor_compress_state_t *state)
 {
   tor_assert(state != NULL);
 
-  if (state->method == GZIP_METHOD || state->method == ZLIB_METHOD)
-    return tor_zlib_compress_state_size(state->u.zlib_state);
+  switch (state->method) {
+    case GZIP_METHOD:
+    case ZLIB_METHOD:
+      return tor_zlib_compress_state_size(state->u.zlib_state);
+
+    case NO_METHOD:
+    case UNKNOWN_METHOD:
+      goto err;
+  }
 
+ err:
   return 0;
 }