From: Ian Jackson <ian@liberator.relativity.greenend.org.uk>
Date: Sun, 9 Aug 2009 15:18:40 +0000 (+0100)
Subject: Actual locking and conflict handling
X-Git-Tag: 3.0~5
X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~yarrgweb/git?p=ypp-sc-tools.web-live.git;a=commitdiff_plain;h=b9ed9d127c61f567c76618b145658763fa7b700e

Actual locking and conflict handling
---

diff --git a/.gitignore b/.gitignore
index d8076a5..5796988 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,3 +12,4 @@ yarrg/yarrg
 
 yarrg/_*.*
 yarrg/OCEAN-*.db
+yarrg/Writer.lock
diff --git a/yarrg/CommodsDatabase.pm b/yarrg/CommodsDatabase.pm
index 4866c85..6f2f627 100644
--- a/yarrg/CommodsDatabase.pm
+++ b/yarrg/CommodsDatabase.pm
@@ -22,10 +22,19 @@
 
 package CommodsDatabase;
 
+# Valid calling sequences:
+#    db_setocean('Midnight')
+#  [ db_filename() => 'OCEAN-Midnight.db'  also OK at any later time ]
+#  [ db_writer() ]                         helpful but not essential
+#    db_connect()
+#  [ db_onconflict(sub { .... }) ]         essential if just dieing is not OK
+#    $dbh->do(...), $dbh->prepare(...), db_doall("stmt;stmt;"), etc.
+
 use strict;
 use warnings;
 
 use DBI;
+use POSIX;
 
 use Commods;
 
@@ -34,8 +43,8 @@ BEGIN {
     our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
     $VERSION     = 1.00;
     @ISA         = qw(Exporter);
-    @EXPORT      = qw(&db_setocean &db_connect $dbh
-		      &db_filename &db_doall);
+    @EXPORT      = qw(&db_setocean &db_writer &db_connect $dbh
+		      &db_filename &db_doall &db_onconflict);
     %EXPORT_TAGS = ( );
 
     @EXPORT_OK   = qw();
@@ -52,12 +61,40 @@ sub db_filename () {
     return $dbfn;
 }
 
+sub db_onconflict (&) {
+    my ($conflictproc) = @_;
+    $dbh->{HandleError}= sub {
+	my ($emsg,$dbh,$val1) = @_;
+	my $native_ecode= $dbh->err();
+	&$conflictproc($emsg) if grep { $_ == $native_ecode } qw(5 6);
+	# 5==SQLITE_BUSY, 6==SQLITE_LOCKED according to the SQLite3
+	# API documentation, .../capi3ref.html#extended-result-codes.
+	return 0; # RaiseError happens next.
+    };
+}
+
+our $writerlockh;
+
+sub db_writer () {
+    my $lockfn= "Writer.lock";
+    $writerlockh= new IO::File "$lockfn", "w" or die "$lockfn $!";
+
+    my $flockall= pack 's!s!LLLLLL', F_WRLCK, SEEK_SET, 0,0,0,0,0,0;
+    # should work everywhere to lock the whole file, provided that
+    # l_type and l_whence are `short int' and come first in that order,
+    # and that start, len and pid are no more than 64 bits each.
+
+    my $r= fcntl($writerlockh, F_SETLKW, $flockall);
+    $r or die "$lockfn fcntl $!";
+}
+
 sub db_connect () {
     $dbh= DBI->connect("dbi:SQLite:$dbfn",'','',
 		       { AutoCommit=>0,
 			 RaiseError=>1, ShowErrorStatement=>1,
 			 unicode=>1 })
 	or die "$dbfn $DBI::errstr ?";
+    # default timeout is 30s which is plenty
 }
 
 sub db_doall ($) {
diff --git a/yarrg/commod-email-processor b/yarrg/commod-email-processor
index bf42c0d..ec4f222 100755
--- a/yarrg/commod-email-processor
+++ b/yarrg/commod-email-processor
@@ -117,7 +117,9 @@ sub main () {
     db_setocean($md{'ocean'});
     my $dbfn= db_filename();
     (stat $dbfn) or die "stat database $dbfn failed $!\n";
+    db_writer();
     db_connect();
+    db_onconflict(sub { print STDERR "temporary failure: @_\n"; exit 75; });
 
     my ($islandid) = $dbh->selectrow_array(
               "SELECT islands.islandid