Add Pidfile to FileWatching (#44367)

Co-authored-by: Jameson Nash <[email protected]>

Co-authored-by: Jameson Nash <[email protected]>

Author: IanButterworth

stdlib/FileWatching/docs/src/index.md

@@ -7,3 +7,31 @@ FileWatching.watch_file
 FileWatching.watch_folder
 FileWatching.unwatch_folder
 ```
+
+# Pidfile
+
+```@meta
+CurrentModule = FileWatching.Pidfile
+```
+
+A simple utility tool for creating advisory pidfiles (lock files).
+
+## Primary Functions
+
+```@docs
+mkpidlock
+close(lock::LockMonitor)
+```
+
+
+## Helper Functions
+
+```@docs
+Pidfile.open_exclusive
+Pidfile.tryopen_exclusive
+Pidfile.write_pidfile
+Pidfile.parse_pidfile
+Pidfile.stale_pidfile
+Pidfile.isvalidpid
+Base.touch(::Pidfile.LockMonitor)
+```

stdlib/FileWatching/src/FileWatching.jl

@@ -16,7 +16,9 @@ export
     FileMonitor,
     FolderMonitor,
     PollingFileWatcher,
-    FDWatcher
+    FDWatcher,
+    # pidfile:
+    mkpidlock
 
 import Base: @handle_as, wait, close, eventloop, notify_error, IOError,
     _sizeof_uv_poll, _sizeof_uv_fs_poll, _sizeof_uv_fs_event, _uv_hook_close, uv_error, _UVError,
@@ -862,4 +864,7 @@ function poll_file(s::AbstractString, interval_seconds::Real=5.007, timeout_s::R
     end
 end
 
+include("pidfile.jl")
+import .Pidfile: mkpidlock
+
 end

stdlib/FileWatching/src/pidfile.jl

@@ -0,0 +1,323 @@
+module Pidfile
+
+
+export mkpidlock
+
+using Base:
+    IOError, UV_EEXIST, UV_ESRCH,
+    Process
+
+using Base.Libc: rand
+
+using Base.Filesystem:
+    File, open, JL_O_CREAT, JL_O_RDWR, JL_O_RDONLY, JL_O_EXCL,
+    rename, samefile, path_separator
+
+using ..FileWatching: watch_file
+using Base.Sys: iswindows
+
+"""
+    mkpidlock([f::Function], at::String, [pid::Cint, proc::Process]; kwopts...)
+
+Create a pidfile lock for the path "at" for the current process
+or the process identified by pid or proc. Can take a function to execute once locked,
+for usage in `do` blocks, after which the lock will be automatically closed. If the lock fails
+and `wait` is false, then an error is thrown.
+
+The lock will be released by either `close`, a `finalizer`, or shortly after `proc` exits.
+Make sure the return value is live through the end of the critical section of
+your program, so the `finalizer` does not reclaim it early.
+
+Optional keyword arguments:
+ - `mode`: file access mode (modified by the process umask). Defaults to world-readable.
+ - `poll_interval`: Specify the maximum time to between attempts (if `watch_file` doesn't work)
+ - `stale_age`: Delete an existing pidfile (ignoring the lock) if its mtime is older than this.
+     The file won't be deleted until 25x longer than this if the pid in the file appears that it may be valid.
+     By default this is disabled (`stale_age` = 0), but a typical recommended value would be about 3-5x an
+     estimated normal completion time.
+ - `refresh`: Keeps a lock from becoming stale by updating the mtime every interval of time that passes.
+     By default, this is set to `stale_age/2`, which is the recommended value.
+ - `wait`: If true, block until we get the lock, if false, raise error if lock fails.
+"""
+function mkpidlock end
+
+
+# mutable only because we want to add a finalizer
+mutable struct LockMonitor
+    const path::String
+    const fd::File
+    const update::Union{Nothing,Timer}
+
+    global function mkpidlock(at::String, pid::Cint; stale_age::Real=0, refresh::Real=stale_age/2, kwopts...)
+        local lock
+        atdir, atname = splitdir(at)
+        isempty(atdir) && (atdir = pwd())
+        at = realpath(atdir) * path_separator * atname
+        fd = open_exclusive(at; stale_age=stale_age, kwopts...)
+        update = nothing
+        try
+            write_pidfile(fd, pid)
+            if refresh > 0
+                # N.b.: to ensure our finalizer works we are careful to capture
+                # `fd` here instead of `lock`.
+                update = Timer(t -> isopen(t) && touch(fd), refresh; interval=refresh)
+            end
+            lock = new(at, fd, update)
+            finalizer(close, lock)
+        catch ex
+            rm(at)
+            close(fd)
+            rethrow(ex)
+        end
+        return lock
+    end
+end
+
+mkpidlock(at::String; kwopts...) = mkpidlock(at, getpid(); kwopts...)
+mkpidlock(f::Function, at::String; kwopts...) = mkpidlock(f, at, getpid(); kwopts...)
+
+function mkpidlock(f::Function, at::String, pid::Cint; kwopts...)
+    lock = mkpidlock(at, pid; kwopts...)
+    try
+        return f()
+    finally
+        close(lock)
+    end
+end
+
+function mkpidlock(at::String, proc::Process; kwopts...)
+    lock = mkpidlock(at, getpid(proc); kwopts...)
+    closer = @async begin
+        wait(proc)
+        close(lock)
+    end
+    isdefined(Base, :errormonitor) && Base.errormonitor(closer)
+    return lock
+end
+
+"""
+    Base.touch(::Pidfile.LockMonitor)
+
+Update the `mtime` on the lock, to indicate it is still fresh.
+
+See also the `refresh` keyword in the [`mkpidlock`](@ref) constructor.
+"""
+Base.touch(lock::LockMonitor) = (touch(lock.fd); lock)
+
+"""
+    write_pidfile(io, pid)
+
+Write our pidfile format to an open IO descriptor.
+"""
+function write_pidfile(io::IO, pid::Cint)
+    print(io, "$pid $(gethostname())")
+end
+
+"""
+    parse_pidfile(file::Union{IO, String}) => (pid, hostname, age)
+
+Attempt to parse our pidfile format,
+replaced an element with (0, "", 0.0), respectively, for any read that failed.
+"""
+function parse_pidfile(io::IO)
+    fields = split(read(io, String), ' ', limit = 2)
+    pid = tryparse(Cuint, fields[1])
+    pid === nothing && (pid = Cuint(0))
+    hostname = (length(fields) == 2) ? fields[2] : ""
+    when = mtime(io)
+    age = time() - when
+    return (pid, hostname, age)
+end
+
+function parse_pidfile(path::String)
+    try
+        existing = open(path, JL_O_RDONLY)
+        try
+            return parse_pidfile(existing)
+        finally
+            close(existing)
+        end
+    catch ex
+        isa(ex, EOFError) || isa(ex, IOError) || rethrow(ex)
+        return (Cuint(0), "", 0.0)
+    end
+end
+
+"""
+    isvalidpid(hostname::String, pid::Cuint) :: Bool
+
+Attempt to conservatively estimate whether pid is a valid process id.
+"""
+function isvalidpid(hostname::AbstractString, pid::Cuint)
+    # can't inspect remote hosts
+    (hostname == "" || hostname == gethostname()) || return true
+    # pid < 0 is never valid (must be a parser error or different OS),
+    # and would have a completely different meaning when passed to kill
+    !iswindows() && pid > typemax(Cint) && return false
+    # (similarly for pid 0)
+    pid == 0 && return false
+    # see if the process id exists by querying kill without sending a signal
+    # and checking if it returned ESRCH (no such process)
+    return ccall(:uv_kill, Cint, (Cuint, Cint), pid, 0) != UV_ESRCH
+end
+
+"""
+    stale_pidfile(path::String, stale_age::Real) :: Bool
+
+Helper function for `open_exclusive` for deciding if a pidfile is stale.
+"""
+function stale_pidfile(path::String, stale_age::Real)
+    pid, hostname, age = parse_pidfile(path)
+    age < -stale_age && @warn "filesystem time skew detected" path=path
+    if age > stale_age
+        if (age > stale_age * 25) || !isvalidpid(hostname, pid)
+            return true
+        end
+    end
+    return false
+end
+
+"""
+    tryopen_exclusive(path::String, mode::Integer = 0o444) :: Union{Void, File}
+
+Try to create a new file for read-write advisory-exclusive access,
+return nothing if it already exists.
+"""
+function tryopen_exclusive(path::String, mode::Integer = 0o444)
+    try
+        return open(path, JL_O_RDWR | JL_O_CREAT | JL_O_EXCL, mode)
+    catch ex
+        (isa(ex, IOError) && ex.code == UV_EEXIST) || rethrow(ex)
+    end
+    return nothing
+end
+
+"""
+    open_exclusive(path::String; mode, poll_interval, stale_age) :: File
+
+Create a new a file for read-write advisory-exclusive access.
+If `wait` is `false` then error out if the lock files exist
+otherwise block until we get the lock.
+
+For a description of the keyword arguments, see [`mkpidlock`](@ref).
+"""
+function open_exclusive(path::String;
+                        mode::Integer = 0o444 #= read-only =#,
+                        poll_interval::Real = 10 #= seconds =#,
+                        wait::Bool = true #= return on failure if false =#,
+                        stale_age::Real = 0 #= disabled =#)
+    # fast-path: just try to open it
+    file = tryopen_exclusive(path, mode)
+    file === nothing || return file
+    if !wait
+        if file === nothing && stale_age > 0
+            if stale_age > 0 && stale_pidfile(path, stale_age)
+                @warn "attempting to remove probably stale pidfile" path=path
+                tryrmopenfile(path)
+            end
+            file = tryopen_exclusive(path, mode)
+        end
+        if file === nothing
+            error("Failed to get pidfile lock for $(repr(path)).")
+        else
+            return file
+        end
+    end
+    # fall-back: wait for the lock
+
+    while true
+        # start the file-watcher prior to checking for the pidfile existence
+        t = @async try
+            watch_file(path, poll_interval)
+        catch ex
+            isa(ex, IOError) || rethrow(ex)
+            sleep(poll_interval) # if the watch failed, convert to just doing a sleep
+        end
+        # now try again to create it
+        file = tryopen_exclusive(path, mode)
+        file === nothing || return file
+        Base.wait(t) # sleep for a bit before trying again
+        if stale_age > 0 && stale_pidfile(path, stale_age)
+            # if the file seems stale, try to remove it before attempting again
+            # set stale_age to zero so we won't attempt again, even if the attempt fails
+            stale_age -= stale_age
+            @warn "attempting to remove probably stale pidfile" path=path
+            tryrmopenfile(path)
+        end
+    end
+end
+
+function _rand_filename(len::Int=4) # modified from Base.Libc
+    slug = Base.StringVector(len)
+    chars = b"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+    for i = 1:len
+        slug[i] = chars[(Libc.rand() % length(chars)) + 1]
+    end
+    return String(slug)
+end
+
+function tryrmopenfile(path::String)
+    # Deleting open file on Windows is a bit hard
+    # if we want to reuse the name immediately after:
+    # we need to first rename it, then delete it.
+    if Sys.iswindows()
+        try
+            local rmpath
+            rmdir, rmname = splitdir(path)
+            while true
+                rmpath = string(rmdir, isempty(rmdir) ? "" : path_separator,
+                    "\$", _rand_filename(), rmname, ".deleted")
+                ispath(rmpath) || break
+            end
+            rename(path, rmpath)
+            path = rmpath
+        catch ex
+            isa(ex, IOError) || rethrow(ex)
+        end
+    end
+    return try
+        rm(path)
+        true
+    catch ex
+        isa(ex, IOError) || rethrow(ex)
+        false
+    end
+end
+
+"""
+    close(lock::LockMonitor)
+
+Release a pidfile lock.
+"""
+function Base.close(lock::LockMonitor)
+    update = lock.update
+    update === nothing || close(update)
+    isopen(lock.fd) || return false
+    removed = false
+    path = lock.path
+    pathstat = try
+            # Windows sometimes likes to return EACCES here,
+            # if the path is in the process of being deleted
+            stat(path)
+        catch ex
+            ex isa IOError || rethrow()
+            removed = ex
+            nothing
+        end
+    if pathstat !== nothing && samefile(stat(lock.fd), pathstat)
+        # try not to delete someone else's lock
+        try
+            rm(path)
+            removed = true
+        catch ex
+            ex isa IOError || rethrow()
+            removed = ex
+        end
+    end
+    close(lock.fd)
+    havelock = removed === true
+    havelock || @warn "failed to remove pidfile on close" path=path removed=removed
+    return havelock
+end
+
+end # module