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1.1 |
=head1 NAME |
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AnyEvent::MCP::File - private state file locking and replacement |
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root |
1.2 |
=head1 DESCRIPTION |
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Internal utility for small private state files shared by MCP processes, |
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such as the workdir ID registry. C<lock> obtains a lock on the current |
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pathname; C<replace> writes a complete new version atomically. The caller |
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owns serialization, validation and the lock's lifetime. |
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For example, after building a new complete file body: |
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use AnyEvent::MCP::File (); |
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my $lock = AnyEvent::MCP::File::lock $path; |
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# read and validate the current state under this lock |
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AnyEvent::MCP::File::replace $path, $new_bytes; |
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close $lock; |
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The lock protects cooperating writers, not other programs that write the |
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file without this protocol. Use L<AnyEvent::MCP::Workdirs> for the workdir |
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registry rather than implementing its update rules here. |
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root |
1.1 |
=head1 FUNCTIONS |
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=over |
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=item $lock = AnyEvent::MCP::File::lock $file |
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Create the parent directory and file if necessary, and lock the current file |
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exclusively. The file must be private and owned by this user. Retries if an |
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atomic replacement changed the pathname while waiting for the lock. |
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Keep the returned handle alive through reading, modifying and replacing |
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the file. All writers must use this locking protocol. |
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=item AnyEvent::MCP::File::replace $file, $bytes |
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Write bytes to a private temporary file beside C<$file>, then atomically |
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rename it over C<$file>. The caller holds the lock. Removes the temporary |
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file on failure. This provides atomic visibility, not crash durability. |
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=back |
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=cut |
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package AnyEvent::MCP::File; |
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use common::sense; |
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use File::Path (); |
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use File::Basename (); |
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use Fcntl qw(O_RDWR O_WRONLY O_CREAT O_EXCL LOCK_EX); |
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use Errno qw(ENOENT EEXIST); |
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sub lock { |
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my ($file) = @_; |
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File::Path::mkpath (File::Basename::dirname ($file), 0, 0700); |
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while (1) { |
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my $fh; |
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sysopen $fh, $file, O_RDWR | O_CREAT | O_EXCL, 0600 or do { |
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die "$file: $!\n" unless $! == EEXIST; |
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sysopen $fh, $file, O_RDWR or die "$file: $!\n"; |
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}; |
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my @stat = stat $fh; |
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die "$file: file must be private and owned by this user\n" if $stat[4] != $< || $stat[2] & 077; |
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flock $fh, LOCK_EX or die "$file: $!\n"; |
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my @current = stat $file; |
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return $fh if @current && $stat[0] == $current[0] && $stat[1] == $current[1]; |
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die "$file: $!\n" unless @current || $! == ENOENT; |
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close $fh; |
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} |
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} |
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sub replace { |
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my ($file, $bytes) = @_; |
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my $temporary = "$file.$$~"; |
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sysopen my $fh, $temporary, O_WRONLY | O_CREAT | O_EXCL, 0600 or die "$temporary: $!\n"; |
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my $ok = eval { |
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print {$fh} $bytes or die "$temporary: $!\n"; |
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close $fh or die "$temporary: $!\n"; |
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rename $temporary, $file or die "$file: $!\n"; |
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1 |
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}; |
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my $error = $@; |
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unlink $temporary unless $ok; |
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die $error unless $ok; |
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} |
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1; |
