pbackup: Support PBACKUP_DB development

Add code to override the sqlite database name via environment variable.
This can help in development, so that code can be tested against a copy
of a real database as an unpriviledged user while avoiding the risk to
block the running server or corrupte the backup data.

bin/pbackup

@@ -150,8 +150,20 @@ sub scan_rsync_log {
 
 our $dbh;
 
+sub get_db_filename {
+    my $db_filename;
+    if (exists $ENV{'PBACKUP_DB'}) {
+        $db_filename = $ENV{'PBACKUP_DB'};
+        -e $db_filename or die "$db_filename: no such file\n";
+    } else {
+        $db_filename = "$ROOT/db/pbackup.db";
+    }
+    return $db_filename;
+}
+
 sub db_open {
-    $dbh=DBI->connect("dbi:SQLite:dbname=$ROOT/db/pbackup.db","","",{
+    my $db_filename = get_db_filename();;
+    $dbh=DBI->connect("dbi:SQLite:dbname=$db_filename","","",{
         AutoCommit=>1,
         PrintError=>0,
         RaiseError=>1,
@@ -1791,7 +1803,8 @@ if ($cmd eq 'status') {
 } elsif ($cmd eq 'fix_stat') {
     cmd_fix_stat();
 } elsif ($cmd eq 'sql') {
-    system('sqlite3','-header',"$ROOT/db/pbackup.db",@ARGV);
+    my $db_filename = get_db_filename();
+    system('sqlite3', '-header', $db_filename, @ARGV);
 } elsif ($cmd eq 'expire') {
     cmd_expire();
 } elsif ($cmd eq 'kill') {