Avoid logging complaints about abandoned connections when using PAM.

For a long time (since commit aed378e) we have had a policy to log
nothing about a connection if the client disconnects when challenged
for a password.  This is because libpq-using clients will typically
do that, and then come back for a new connection attempt once they've
collected a password from their user, so that logging the abandoned
connection attempt will just result in log spam.  However, this did
not work well for PAM authentication: the bottom-level function
pam_passwd_conv_proc() was on board with it, but we logged messages
at higher levels anyway, for lack of any reporting mechanism.
Add a flag and tweak the logic so that the case is silent, as it is
for other password-using auth mechanisms.

Per complaint from Yoann La Cancellera.  It's been like this for awhile,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/CACP=ajbrFFYUrLyJBLV8=q+eNCapa1xDEyvXhMoYrNphs-xqPw@mail.gmail.com

Author: tglsfdc

src/backend/libpq/auth.c

@@ -110,6 +110,7 @@ static const char *pam_passwd = NULL;	/* Workaround for Solaris 2.6
 										 * brokenness */
 static Port *pam_port_cludge;	/* Workaround for passing "Port *port" into
 								 * pam_passwd_conv_proc */
+static bool pam_no_password;	/* For detecting no-password-given */
 #endif							/* USE_PAM */
 
 
@@ -2099,8 +2100,10 @@ pam_passwd_conv_proc(int num_msg, const struct pam_message **msg,
 					{
 						/*
 						 * Client didn't want to send password.  We
-						 * intentionally do not log anything about this.
+						 * intentionally do not log anything about this,
+						 * either here or at higher levels.
 						 */
+						pam_no_password = true;
 						goto fail;
 					}
 				}
@@ -2159,6 +2162,7 @@ CheckPAMAuth(Port *port, const char *user, const char *password)
 	 */
 	pam_passwd = password;
 	pam_port_cludge = port;
+	pam_no_password = false;
 
 	/*
 	 * Set the application data portion of the conversation struct.  This is
@@ -2244,22 +2248,26 @@ CheckPAMAuth(Port *port, const char *user, const char *password)
 
 	if (retval != PAM_SUCCESS)
 	{
-		ereport(LOG,
-				(errmsg("pam_authenticate failed: %s",
-						pam_strerror(pamh, retval))));
+		/* If pam_passwd_conv_proc saw EOF, don't log anything */
+		if (!pam_no_password)
+			ereport(LOG,
+					(errmsg("pam_authenticate failed: %s",
+							pam_strerror(pamh, retval))));
 		pam_passwd = NULL;		/* Unset pam_passwd */
-		return STATUS_ERROR;
+		return pam_no_password ? STATUS_EOF : STATUS_ERROR;
 	}
 
 	retval = pam_acct_mgmt(pamh, 0);
 
 	if (retval != PAM_SUCCESS)
 	{
-		ereport(LOG,
-				(errmsg("pam_acct_mgmt failed: %s",
-						pam_strerror(pamh, retval))));
+		/* If pam_passwd_conv_proc saw EOF, don't log anything */
+		if (!pam_no_password)
+			ereport(LOG,
+					(errmsg("pam_acct_mgmt failed: %s",
+							pam_strerror(pamh, retval))));
 		pam_passwd = NULL;		/* Unset pam_passwd */
-		return STATUS_ERROR;
+		return pam_no_password ? STATUS_EOF : STATUS_ERROR;
 	}
 
 	retval = pam_end(pamh, retval);