103 lines
3.1 KiB
Go
103 lines
3.1 KiB
Go
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// Copyright (c) 2018 David Crawshaw <david@zentus.com>
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// Copyright (c) 2021 Ross Light <ross@zombiezen.com>
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//
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// Permission to use, copy, modify, and distribute this software for any
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// purpose with or without fee is hereby granted, provided that the above
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// copyright notice and this permission notice appear in all copies.
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//
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// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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//
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// SPDX-License-Identifier: ISC
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package sqlite
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import (
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"fmt"
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"sync"
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"unsafe"
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"modernc.org/libc"
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"modernc.org/libc/sys/types"
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lib "modernc.org/sqlite/lib"
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)
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// See https://sqlite.org/unlock_notify.html for detailed explanation.
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// unlockNote is a C-allocated struct used as a condition variable.
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type unlockNote struct {
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mu sync.Mutex
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wait sync.Mutex // held while fired == false
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fired bool
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}
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func allocUnlockNote(tls *libc.TLS) (uintptr, error) {
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ptr := libc.Xcalloc(tls, 1, types.Size_t(unsafe.Sizeof(unlockNote{})))
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if ptr == 0 {
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return 0, fmt.Errorf("out of memory for unlockNote")
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}
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un := (*unlockNote)(unsafe.Pointer(ptr))
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un.wait.Lock()
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return ptr, nil
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}
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func fireUnlockNote(tls *libc.TLS, ptr uintptr) {
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un := (*unlockNote)(unsafe.Pointer(ptr))
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un.mu.Lock()
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if !un.fired {
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un.fired = true
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un.wait.Unlock()
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}
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un.mu.Unlock()
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}
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func unlockNotifyCallback(tls *libc.TLS, apArg uintptr, nArg int32) {
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for ; nArg > 0; nArg-- {
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fireUnlockNote(tls, *(*uintptr)(unsafe.Pointer(apArg)))
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// apArg is a C array of pointers.
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apArg += unsafe.Sizeof(uintptr(0))
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}
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}
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func waitForUnlockNotify(tls *libc.TLS, db uintptr, unPtr uintptr) ResultCode {
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un := (*unlockNote)(unsafe.Pointer(unPtr))
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if un.fired {
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un.wait.Lock()
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}
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un.fired = false
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// The following is a conversion from function value to uintptr. It assumes
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// the memory representation described in https://golang.org/s/go11func.
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//
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// It does this by doing the following in order:
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// 1) Create a Go struct containing a pointer to a pointer to
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// unlockNotifyCallback. It is assumed that the pointer to
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// unlockNotifyCallback will be stored in the read-only data section and
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// thus will not move.
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// 2) Convert the pointer to the Go struct to a pointer to uintptr through
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// unsafe.Pointer. This is permitted via Rule #1 of unsafe.Pointer.
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// 3) Dereference the pointer to uintptr to obtain the function value as a
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// uintptr. This is safe as long as function values are passed as pointers.
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cbPtr := *(*uintptr)(unsafe.Pointer(&struct {
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f func(*libc.TLS, uintptr, int32)
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}{unlockNotifyCallback}))
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res := ResultCode(lib.Xsqlite3_unlock_notify(tls, db, cbPtr, unPtr))
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if res == ResultOK {
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un.mu.Lock()
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fired := un.fired
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un.mu.Unlock()
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if !fired {
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un.wait.Lock()
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un.wait.Unlock()
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}
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}
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return res
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}
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