msys2-runtime: restore fast path for current user primary group

Commit a5bcfe6 removed an optimization that fetches the
default group from the current user token, as it is sometimes
not accurate such as when groups like the builtin
Administrators group is the primary group.

However, removing this optimization causes extremely poor
performance when connected to some Active Directory
environments.

Restored this optimization as the default behaviour, and
added a `group: db-accurate` option to `nsswitch.conf` that
can be used to disable the optimization in cases where
accurate group information is required.

This fixes git-for-windows/git#4459

Signed-off-by: Richard Glidden <richard@glidden.org>

winsup/cygwin/include/sys/cygwin.h

@@ -219,7 +219,8 @@ enum
 enum
 {
   NSS_SRC_FILES = 1,
-  NSS_SRC_DB = 2
+  NSS_SRC_DB = 2,
+  NSS_SRC_DB_ACCURATE = 4
 };
 
 /* Enumeration source constants for CW_SETENT called from mkpasswd/mkgroup. */

winsup/cygwin/local_includes/cygheap.h

@@ -454,6 +454,7 @@ class cygheap_pwdgrp
   inline int  nss_pwd_src () const { return pwd_src; } /* CW_GETNSS_PWD_SRC */
   inline bool nss_grp_files () const { return !!(grp_src & NSS_SRC_FILES); }
   inline bool nss_grp_db () const { return !!(grp_src & NSS_SRC_DB); }
+  inline bool nss_grp_db_accurate () const { return !!(grp_src & NSS_SRC_DB_ACCURATE); }
   inline int  nss_grp_src () const { return grp_src; } /* CW_GETNSS_GRP_SRC */
   inline bool nss_cygserver_caching () const { return caching; }
   inline void nss_disable_cygserver_caching () { caching = false; }

winsup/cygwin/uinfo.cc

@@ -637,6 +637,11 @@ cygheap_pwdgrp::nss_init_line (const char *line)
 		    *src |= NSS_SRC_DB;
 		    c += 2;
 		  }
+		else if (NSS_CMP ("db-accurate"))
+		  {
+		    *src |= NSS_SRC_DB | NSS_SRC_DB_ACCURATE;
+		    c += 11;
+		  }
 		else
 		  {
 		    c += strcspn (c, " \t");
@@ -1906,6 +1911,7 @@ pwdgrp::fetch_account_from_windows (fetch_user_arg_t &arg, cyg_ldap *pldap)
   gid_t gid = ILLEGAL_GID;
   bool is_domain_account = true;
   PCWSTR domain = NULL;
+  bool get_default_group_from_current_user_token = false;
   char *shell = NULL;
   char *home = NULL;
   char *gecos = NULL;
@@ -2371,9 +2377,19 @@ pwdgrp::fetch_account_from_windows (fetch_user_arg_t &arg, cyg_ldap *pldap)
 	    uid = posix_offset + sid_sub_auth_rid (sid);
 	  if (!is_group () && acc_type == SidTypeUser)
 	    {
-	      /* Default primary group.  Make the educated guess that the user
-		 is in group "Domain Users" or "None". */
-	      gid = posix_offset + DOMAIN_GROUP_RID_USERS;
+	      /* Default primary group.  If the sid is the current user, and
+		 we are not configured for accurate mode, fetch
+		 the default group from the current user token, otherwise make
+		 the educated guess that the user is in group "Domain Users"
+		 or "None". */
+	      if (!cygheap->pg.nss_grp_db_accurate() && sid == cygheap->user.sid ())
+		{
+		  get_default_group_from_current_user_token = true;
+		  gid = posix_offset
+			+ sid_sub_auth_rid (cygheap->user.groups.pgsid);
+		}
+	      else
+		gid = posix_offset + DOMAIN_GROUP_RID_USERS;
 	    }
 
 	  if (is_domain_account)
@@ -2384,9 +2400,11 @@ pwdgrp::fetch_account_from_windows (fetch_user_arg_t &arg, cyg_ldap *pldap)
 	      /* On AD machines, use LDAP to fetch domain account infos. */
 	      if (cygheap->dom.primary_dns_name ())
 		{
-		  /* Fetch primary group from AD and overwrite the one we
-		     just guessed above. */
-		  if (cldap->fetch_ad_account (sid, false, domain))
+		  /* For the current user we got correctly cased username and
+		     the primary group via process token.  For any other user
+		     we fetch it from AD and overwrite it. */
+		  if (!get_default_group_from_current_user_token
+		      && cldap->fetch_ad_account (sid, false, domain))
 		    {
 		      if ((val = cldap->get_account_name ()))
 			wcscpy (name, val);

winsup/doc/ntsec.xml

@@ -930,7 +930,16 @@ The two lines starting with the keywords <literal>passwd:</literal> and
 information from.  <literal>files</literal> means, fetch the information
 from the corresponding file in the /etc directory.  <literal>db</literal>
 means, fetch the information from the Windows account databases, the SAM
-for local accounts, Active Directory for domain account.  Examples:
+for local accounts, Active Directory for domain account.  For the current
+user, the default group is obtained from the current user token to avoid
+additional lookups to the group database. <literal>db-accurate</literal>
+is only valid on <literal>group:</literal> line, and performs the same 
+lookups as the <literal>db</literal> option, but disables using the
+current user token to retrieve the default group as this optimization
+is not accurate in all cases.  For example, if you run a native process
+with the primary group set to the Administrators builtin group, the 
+<literal>db</literal> option will return a non-existent group as primary
+group. Examples:
 </para>
 
 <screen>
@@ -949,6 +958,15 @@ Read passwd entries only from /etc/passwd.
 Read group entries only from SAM/AD.
 </para>
 
+<screen>
+  group: db-accurate
+</screen>
+
+<para>
+Read group entries only from SAM/AD. Force the use of the group database
+for the current user.
+</para>
+
 <screen>
   group: files # db
 </screen>