1 /*
2 * Copyright (C) 2011 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #include "scoped_flock.h"
18
19 #include <sys/file.h>
20 #include <sys/stat.h>
21
22 #include <android-base/logging.h>
23 #include <android-base/stringprintf.h>
24
25 #include "file_utils.h"
26 #include "unix_file/fd_file.h"
27
28 namespace art {
29
30 using android::base::StringPrintf;
31
Open(const char * filename,std::string * error_msg)32 /* static */ ScopedFlock LockedFile::Open(const char* filename, std::string* error_msg) {
33 return Open(filename, O_CREAT | O_RDWR, true, error_msg);
34 }
35
Open(const char * filename,int flags,bool block,std::string * error_msg)36 /* static */ ScopedFlock LockedFile::Open(const char* filename, int flags, bool block,
37 std::string* error_msg) {
38 #ifdef _WIN32
39 // TODO: implement file locking for Windows.
40 UNUSED(filename);
41 UNUSED(flags);
42 UNUSED(block);
43 *error_msg = "flock is unsupported on Windows";
44 return nullptr;
45 #else
46 while (true) {
47 // NOTE: We don't check usage here because the ScopedFlock should *never* be
48 // responsible for flushing its underlying FD. Its only purpose should be
49 // to acquire a lock, and the unlock / close in the corresponding
50 // destructor. Callers should explicitly flush files they're writing to if
51 // that is the desired behaviour.
52 std::unique_ptr<File> file(OS::OpenFileWithFlags(filename, flags, /* auto_flush= */ false));
53 if (file.get() == nullptr) {
54 *error_msg = StringPrintf("Failed to open file '%s': %s", filename, strerror(errno));
55 return nullptr;
56 }
57
58 int operation = block ? LOCK_EX : (LOCK_EX | LOCK_NB);
59 int flock_result = TEMP_FAILURE_RETRY(flock(file->Fd(), operation));
60 if (flock_result == EWOULDBLOCK) {
61 // File is locked by someone else and we are required not to block;
62 return nullptr;
63 }
64 if (flock_result != 0) {
65 *error_msg = StringPrintf("Failed to lock file '%s': %s", filename, strerror(errno));
66 return nullptr;
67 }
68 struct stat fstat_stat;
69 int fstat_result = TEMP_FAILURE_RETRY(fstat(file->Fd(), &fstat_stat));
70 if (fstat_result != 0) {
71 *error_msg = StringPrintf("Failed to fstat file '%s': %s", filename, strerror(errno));
72 return nullptr;
73 }
74 struct stat stat_stat;
75 int stat_result = TEMP_FAILURE_RETRY(stat(filename, &stat_stat));
76 if (stat_result != 0) {
77 PLOG(WARNING) << "Failed to stat, will retry: " << filename;
78 // ENOENT can happen if someone racing with us unlinks the file we created so just retry.
79 if (block) {
80 continue;
81 } else {
82 // Note that in theory we could race with someone here for a long time and end up retrying
83 // over and over again. This potential behavior does not fit well in the non-blocking
84 // semantics. Thus, if we are not require to block return failure when racing.
85 return nullptr;
86 }
87 }
88 if (fstat_stat.st_dev != stat_stat.st_dev || fstat_stat.st_ino != stat_stat.st_ino) {
89 LOG(WARNING) << "File changed while locking, will retry: " << filename;
90 if (block) {
91 continue;
92 } else {
93 // See comment above.
94 return nullptr;
95 }
96 }
97
98 return ScopedFlock(new LockedFile(std::move((*file.get()))));
99 }
100 #endif
101 }
102
DupOf(const int fd,const std::string & path,const bool read_only_mode,std::string * error_msg)103 ScopedFlock LockedFile::DupOf(const int fd, const std::string& path,
104 const bool read_only_mode, std::string* error_msg) {
105 #ifdef _WIN32
106 // TODO: implement file locking for Windows.
107 UNUSED(fd);
108 UNUSED(path);
109 UNUSED(read_only_mode);
110 *error_msg = "flock is unsupported on Windows.";
111 return nullptr;
112 #else
113 // NOTE: We don't check usage here because the ScopedFlock should *never* be
114 // responsible for flushing its underlying FD. Its only purpose should be
115 // to acquire a lock, and the unlock / close in the corresponding
116 // destructor. Callers should explicitly flush files they're writing to if
117 // that is the desired behaviour.
118 ScopedFlock locked_file(
119 new LockedFile(DupCloexec(fd), path, /* check_usage= */ false, read_only_mode));
120 if (locked_file->Fd() == -1) {
121 *error_msg = StringPrintf("Failed to duplicate open file '%s': %s",
122 locked_file->GetPath().c_str(), strerror(errno));
123 return nullptr;
124 }
125 if (0 != TEMP_FAILURE_RETRY(flock(locked_file->Fd(), LOCK_EX))) {
126 *error_msg = StringPrintf(
127 "Failed to lock file '%s': %s", locked_file->GetPath().c_str(), strerror(errno));
128 return nullptr;
129 }
130
131 return locked_file;
132 #endif
133 }
134
ReleaseLock()135 void LockedFile::ReleaseLock() {
136 #ifndef _WIN32
137 if (this->Fd() != -1) {
138 int flock_result = TEMP_FAILURE_RETRY(flock(this->Fd(), LOCK_UN));
139 if (flock_result != 0) {
140 // Only printing a warning is okay since this is only used with either:
141 // 1) a non-blocking Init call, or
142 // 2) as a part of a seperate binary (eg dex2oat) which has it's own timeout logic to prevent
143 // deadlocks.
144 // This means we can be sure that the warning won't cause a deadlock.
145 PLOG(WARNING) << "Unable to unlock file " << this->GetPath();
146 }
147 }
148 #endif
149 }
150
151 } // namespace art
152