Allow deriving the current user's home directory via the HOME variable

This patch hails from Git for Windows (where the Cygwin runtime is used
in the form of a slightly modified MSYS2 runtime), where it is a
well-established technique to let the `$HOME` variable define where the
current user's home directory is, falling back to `$HOMEDRIVE$HOMEPATH`
and `$USERPROFILE`.

The idea is that we want to share user-specific settings between
programs, whether they be Cygwin, MSYS2 or not.  Unfortunately, we
cannot blindly activate the "db_home: windows" setting because in some
setups, the user's home directory is set to a hidden directory via an
UNC path (\\share\some\hidden\folder$) -- something many programs
cannot handle correctly, e.g. `cmd.exe` and other native Windows
applications that users want to employ as Git helpers.

The established technique is to allow setting the user's home directory
via the environment variables mentioned above: `$HOMEDRIVE$HOMEPATH` or
`$USERPROFILE`.  This has the additional advantage that it is much
faster than querying the Windows user database.

Of course this scheme needs to be opt-in.  For that reason, it needs
to be activated explicitly via `db_home: env` in `/etc/nsswitch.conf`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/local_includes/cygheap.h

@@ -358,7 +358,8 @@ public:
     NSS_SCHEME_UNIX,
     NSS_SCHEME_DESC,
     NSS_SCHEME_PATH,
-    NSS_SCHEME_FREEATTR
+    NSS_SCHEME_FREEATTR,
+    NSS_SCHEME_ENV
   };
   struct nss_scheme_t {
     nss_scheme_method	method;

winsup/cygwin/uinfo.cc

@@ -733,6 +733,8 @@ cygheap_pwdgrp::nss_init_line (const char *line)
 		    scheme[idx].method = NSS_SCHEME_UNIX;
 		  else if (NSS_CMP ("desc"))
 		    scheme[idx].method = NSS_SCHEME_DESC;
+		  else if (NSS_CMP ("env"))
+		    scheme[idx].method = NSS_SCHEME_ENV;
 		  else if (NSS_NCMP ("/"))
 		    {
 		      const char *e = c + strcspn (c, " \t");
@@ -921,6 +923,42 @@ fetch_from_path (cyg_ldap *pldap, PUSER_INFO_3 ui, cygpsid &sid, PCWSTR str,
   return ret;
 }
 
+static char *
+fetch_home_env (void)
+{
+  /* If `HOME` is set, prefer it */
+  const char *home = getenv ("HOME");
+  if (home)
+    return strdup (home);
+
+  /* If `HOME` is unset, fall back to `HOMEDRIVE``HOMEPATH`
+     (without a directory separator, as `HOMEPATH` starts with one). */
+  const char *home_drive = getenv ("HOMEDRIVE");
+  if (home_drive)
+    {
+      const char *home_path = getenv ("HOMEPATH");
+      if (home_path)
+	{
+	  tmp_pathbuf tp;
+	  char *p = tp.c_get (), *q;
+
+	  // concatenate HOMEDRIVE and HOMEPATH
+	  q = stpncpy (p, home_drive, NT_MAX_PATH);
+	  strlcpy (q, home_path, NT_MAX_PATH - (q - p));
+	  return (char *) cygwin_create_path (CCP_WIN_A_TO_POSIX, p);
+	}
+    }
+
+  /* If neither `HOME` nor `HOMEDRIVE``HOMEPATH` are set, fall back
+     to `USERPROFILE`; In corporate setups, this might point to a
+     disconnected network share, hence this is the last fall back. */
+  home = getenv ("USERPROFILE");
+  if (home)
+    return (char *) cygwin_create_path (CCP_WIN_A_TO_POSIX, home);
+
+  return NULL;
+}
+
 char *
 cygheap_pwdgrp::get_home (cyg_ldap *pldap, cygpsid &sid, PCWSTR dom,
 			  PCWSTR dnsdomain, PCWSTR name, bool full_qualified)
@@ -980,6 +1018,10 @@ cygheap_pwdgrp::get_home (cyg_ldap *pldap, cygpsid &sid, PCWSTR dom,
 		}
 	    }
 	  break;
+	case NSS_SCHEME_ENV:
+	  if (RtlEqualSid (sid, cygheap->user.sid ()))
+	    home = fetch_home_env ();
+	  break;
 	}
     }
   return home;
@@ -1012,6 +1054,10 @@ cygheap_pwdgrp::get_home (PUSER_INFO_3 ui, cygpsid &sid, PCWSTR dom,
 	  home = fetch_from_path (NULL, ui, sid, home_scheme[idx].attrib,
 				  dom, NULL, name, full_qualified);
 	  break;
+	case NSS_SCHEME_ENV:
+	  if (RtlEqualSid (sid, cygheap->user.sid ()))
+	    home = fetch_home_env ();
+	  break;
 	}
     }
   return home;
@@ -1031,6 +1077,7 @@ cygheap_pwdgrp::get_shell (cyg_ldap *pldap, cygpsid &sid, PCWSTR dom,
 	case NSS_SCHEME_FALLBACK:
 	  return NULL;
 	case NSS_SCHEME_WINDOWS:
+	case NSS_SCHEME_ENV:
 	  break;
 	case NSS_SCHEME_CYGWIN:
 	  if (pldap->fetch_ad_account (sid, false, dnsdomain))
@@ -1095,6 +1142,7 @@ cygheap_pwdgrp::get_shell (PUSER_INFO_3 ui, cygpsid &sid, PCWSTR dom,
 	case NSS_SCHEME_CYGWIN:
 	case NSS_SCHEME_UNIX:
 	case NSS_SCHEME_FREEATTR:
+	case NSS_SCHEME_ENV:
 	  break;
 	case NSS_SCHEME_DESC:
 	  if (ui)
@@ -1176,6 +1224,8 @@ cygheap_pwdgrp::get_gecos (cyg_ldap *pldap, cygpsid &sid, PCWSTR dom,
 		sys_wcstombs_alloc (&gecos, HEAP_NOTHEAP, val);
 	    }
 	  break;
+	case NSS_SCHEME_ENV:
+	  break;
 	}
     }
   if (gecos)
@@ -1202,6 +1252,7 @@ cygheap_pwdgrp::get_gecos (PUSER_INFO_3 ui, cygpsid &sid, PCWSTR dom,
 	case NSS_SCHEME_CYGWIN:
 	case NSS_SCHEME_UNIX:
 	case NSS_SCHEME_FREEATTR:
+	case NSS_SCHEME_ENV:
 	  break;
 	case NSS_SCHEME_DESC:
 	  if (ui)

winsup/doc/ntsec.xml

@@ -1203,6 +1203,17 @@ schemata are the following:
 	      See <xref linkend="ntsec-mapping-nsswitch-desc"></xref>
 	      for a more detailed description.</listitem>
   </varlistentry>
+  <varlistentry>
+    <term><literal>env</literal></term>
+    <listitem>Derives the home directory of the current user from the
+	      environment variable <literal>HOME</literal> (falling back to
+	      <literal>HOMEDRIVE\HOMEPATH</literal> and
+	      <literal>USERPROFILE</literal>, in that order).  This is faster
+	      than the <term><literal>windows</literal></term> schema at the
+	      expense of determining only the current user's home directory
+	      correctly.  This schema is skipped for any other account.
+	      </listitem>
+  </varlistentry>
 </variablelist>
 
 <para>
@@ -1335,6 +1346,17 @@ of each schema when used with <literal>db_home:</literal>
 	      See <xref linkend="ntsec-mapping-nsswitch-desc"></xref>
 	      for a detailed description.</listitem>
   </varlistentry>
+  <varlistentry>
+    <term><literal>env</literal></term>
+    <listitem>Derives the home directory of the current user from the
+	      environment variable <literal>HOME</literal> (falling back to
+	      <literal>HOMEDRIVE\HOMEPATH</literal> and
+	      <literal>USERPROFILE</literal>, in that order).  This is faster
+	      than the <term><literal>windows</literal></term> schema at the
+	      expense of determining only the current user's home directory
+	      correctly.  This schema is skipped for any other account.
+	      </listitem>
+  </varlistentry>
   <varlistentry>
     <term><literal>@ad_attribute</literal></term>
     <listitem>AD only: The user's home directory is set to the path given