d8e19cd79a
This adds a P/V style semaphore mechanism to the resource graph. This enables the user to specify a number of "id:count" tags associated with each resource which will reduce the parallelism of the CheckApply operation to that maximum count. This is particularly interesting because (assuming I'm not mistaken) the implementation is dead-lock free assuming that no individual resource permanently ever blocks during execution! I don't have a formal proof of this, but I was able to convince myself on paper that it was the case. An actual proof that N P/V counting semaphores in a DAG won't ever dead-lock would be particularly welcome! Hint: the trick is to acquire them in alphabetical order while respecting the DAG flow. Disclaimer, this assumes that the lock count is always > 0 of course.
79 lines
2.3 KiB
Go
79 lines
2.3 KiB
Go
// Mgmt
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// Copyright (C) 2013-2017+ James Shubin and the project contributors
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// Written by James Shubin <james@shubin.ca> and the project contributors
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Affero General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Affero General Public License for more details.
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//
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// You should have received a copy of the GNU Affero General Public License
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// along with this program. If not, see <http://www.gnu.org/licenses/>.
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package pgraph
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import (
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"fmt"
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"sort"
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"strconv"
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"strings"
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"github.com/purpleidea/mgmt/util/semaphore"
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multierr "github.com/hashicorp/go-multierror"
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)
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// SemaSep is the trailing separator to split the semaphore id from the size.
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const SemaSep = ":"
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// SemaLock acquires the list of semaphores in the graph.
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func (g *Graph) SemaLock(semas []string) error {
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var reterr error
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sort.Strings(semas) // very important to avoid deadlock in the dag!
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for _, id := range semas {
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size := 1 // default semaphore size
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// valid id's include "some_id", "hello:42" and ":13"
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if index := strings.LastIndex(id, SemaSep); index > -1 && (len(id)-index+len(SemaSep)) >= 1 {
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// NOTE: we only allow size > 0 here!
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if i, err := strconv.Atoi(id[index+len(SemaSep):]); err == nil && i > 0 {
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size = i
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}
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}
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sema, ok := g.semas[id] // lookup
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if !ok {
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g.semas[id] = semaphore.NewSemaphore(size)
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sema = g.semas[id]
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}
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if err := sema.P(1); err != nil { // lock!
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reterr = multierr.Append(reterr, err) // list of errors
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}
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}
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return reterr
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}
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// SemaUnlock releases the list of semaphores in the graph.
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func (g *Graph) SemaUnlock(semas []string) error {
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var reterr error
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sort.Strings(semas) // unlock in the same order to remove partial locks
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for _, id := range semas {
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sema, ok := g.semas[id] // lookup
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if !ok {
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// programming error!
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panic(fmt.Sprintf("graph: sema: %s does not exist", id))
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}
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if err := sema.V(1); err != nil { // unlock!
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reterr = multierr.Append(reterr, err) // list of errors
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}
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}
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return reterr
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}
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