[3.15] gh-159739: Propagate fallback getaddrinfo allocation failures (GH-159984) (#160099)

On builds using CPython's fallback `getaddrinfo()`, an allocation failure while constructing address results can return success with a null result, and the canonical-name path can ignore a helper error. `socket.getaddrinfo()` can then return an empty list, other callers can crash when they dereference the null result, and partially built address lists may leak. Set `EAI_MEMORY`, propagate helper errors, and keep the partial list available for cleanup.
(cherry picked from commit 7c39e8f)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>

Author: miss-islington

Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.9ef89c.rst

@@ -0,0 +1,2 @@
+Fix allocation error handling in the fallback implementation of
+``getaddrinfo()``.

Modules/getaddrinfo.c

@@ -171,7 +171,10 @@ if (pai->ai_flags & AI_CANONNAME) {\
     char *p;\
     if (((ai) = (struct addrinfo *)malloc(sizeof(struct addrinfo) +\
                                           ((gai_afd)->a_socklen)))\
-        == NULL) goto free;\
+        == NULL) {\
+        error = EAI_MEMORY;\
+        goto free;\
+    }\
     memcpy(ai, pai, sizeof(struct addrinfo));\
     (ai)->ai_addr = (struct sockaddr *)((ai) + 1);\
     memset((ai)->ai_addr, 0, (gai_afd)->a_socklen);\
@@ -186,7 +189,10 @@ if (pai->ai_flags & AI_CANONNAME) {\
     char *p;\
     if (((ai) = (struct addrinfo *)malloc(sizeof(struct addrinfo) +\
                                           ((gai_afd)->a_socklen)))\
-        == NULL) goto free;\
+        == NULL) {\
+        error = EAI_MEMORY;\
+        goto free;\
+    }\
     memcpy(ai, pai, sizeof(struct addrinfo));\
     (ai)->ai_addr = (struct sockaddr *)((ai) + 1);\
     memset((ai)->ai_addr, 0, (gai_afd)->a_socklen);\
@@ -413,8 +419,8 @@ getaddrinfo(const char*hostname, const char*servname,
                  */
             }
             cur = cur->ai_next;
+            top = sentinel.ai_next;
         }
-        top = sentinel.ai_next;
         if (top)
             goto good;
         else
@@ -463,7 +469,10 @@ getaddrinfo(const char*hostname, const char*servname,
                  * translation function, and it looks strange
                  * that we do addr->name translation here.
                  */
-                get_name(pton, &gai_afdl[i], &top, pton, pai, port);
+                error = get_name(pton, &gai_afdl[i], &top, pton, pai, port);
+                if (error != SUCCESS) {
+                    goto free;
+                }
                 goto good;
             } else
                 ERR(EAI_FAMILY);                        /*xxx*/