Allow parallel workers to retrieve some data from Port

This commit moves authn_id into a new global structure called
ClientConnectionInfo (mapping to a MyClientConnectionInfo for each
backend) which is intended to hold all the client information that
should be shared between the backend and any of its parallel workers,
access for extensions and triggers being the primary use case.  There is
no need to push all the data of Port to the workers, and authn_id is
quite a generic concept so using a separate structure provides the best
balance (the name of the structure has been suggested by Robert Haas).

While on it, and per discussion as this would be useful for a potential
SYSTEM_USER that can be accessed through parallel workers, a second
field is added for the authentication method, copied directly from
Port.

ClientConnectionInfo is serialized and restored using a new parallel
key and a structure tracks the length of the authn_id, making the
addition of more fields straight-forward.

Author: Jacob Champion
Reviewed-by: Bertrand Drouvot, Stephen Frost, Robert Haas, Tom Lane,
Michael Paquier, Julien Rouhaud
Discussion: https://postgr.es/m/793d990837ae5c06a558d58d62de9378ab525d83.camel@vmware.com

src/backend/utils/init/miscinit.c

@@ -936,6 +936,99 @@ GetUserNameFromId(Oid roleid, bool noerr)
 	return result;
 }
 
+/* ------------------------------------------------------------------------
+ *				Client connection state shared with parallel workers
+ *
+ * ClientConnectionInfo contains pieces of information about the client that
+ * need to be synced to parallel workers when they initialize.
+ *-------------------------------------------------------------------------
+ */
+
+ClientConnectionInfo MyClientConnectionInfo;
+
+/*
+ * Intermediate representation of ClientConnectionInfo for easier
+ * serialization.  Variable-length fields are allocated right after this
+ * header.
+ */
+typedef struct SerializedClientConnectionInfo
+{
+	int32		authn_id_len;	/* strlen(authn_id), or -1 if NULL */
+	UserAuth	auth_method;
+} SerializedClientConnectionInfo;
+
+/*
+ * Calculate the space needed to serialize MyClientConnectionInfo.
+ */
+Size
+EstimateClientConnectionInfoSpace(void)
+{
+	Size		size = 0;
+
+	size = add_size(size, sizeof(SerializedClientConnectionInfo));
+
+	if (MyClientConnectionInfo.authn_id)
+		size = add_size(size, strlen(MyClientConnectionInfo.authn_id) + 1);
+
+	return size;
+}
+
+/*
+ * Serialize MyClientConnectionInfo for use by parallel workers.
+ */
+void
+SerializeClientConnectionInfo(Size maxsize, char *start_address)
+{
+	SerializedClientConnectionInfo serialized = {0};
+
+	serialized.authn_id_len = -1;
+	serialized.auth_method = MyClientConnectionInfo.auth_method;
+
+	if (MyClientConnectionInfo.authn_id)
+		serialized.authn_id_len = strlen(MyClientConnectionInfo.authn_id);
+
+	/* Copy serialized representation to buffer */
+	Assert(maxsize >= sizeof(serialized));
+	memcpy(start_address, &serialized, sizeof(serialized));
+
+	maxsize -= sizeof(serialized);
+	start_address += sizeof(serialized);
+
+	/* Copy authn_id into the space after the struct */
+	if (serialized.authn_id_len >= 0)
+	{
+		Assert(maxsize >= (serialized.authn_id_len + 1));
+		memcpy(start_address,
+			   MyClientConnectionInfo.authn_id,
+		/* include the NULL terminator to ease deserialization */
+			   serialized.authn_id_len + 1);
+	}
+}
+
+/*
+ * Restore MyClientConnectionInfo from its serialized representation.
+ */
+void
+RestoreClientConnectionInfo(char *conninfo)
+{
+	SerializedClientConnectionInfo serialized;
+
+	memcpy(&serialized, conninfo, sizeof(serialized));
+
+	/* Copy the fields back into place */
+	MyClientConnectionInfo.authn_id = NULL;
+	MyClientConnectionInfo.auth_method = serialized.auth_method;
+
+	if (serialized.authn_id_len >= 0)
+	{
+		char	   *authn_id;
+
+		authn_id = conninfo + sizeof(serialized);
+		MyClientConnectionInfo.authn_id = MemoryContextStrdup(TopMemoryContext,
+															  authn_id);
+	}
+}
+
 
 /*-------------------------------------------------------------------------
  *				Interlock-file support