lix/src/libstore/db.cc

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#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
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#include <memory>
#include "db.hh"
#include "util.hh"
#include "pathlocks.hh"
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/* Wrapper class to ensure proper destruction. */
class DestroyDbc
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{
Dbc * dbc;
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public:
DestroyDbc(Dbc * _dbc) : dbc(_dbc) { }
~DestroyDbc() { dbc->close(); /* close() frees dbc */ }
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};
static void rethrow(DbException & e)
{
throw Error(e.what());
}
Transaction::Transaction()
: txn(0)
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{
}
Transaction::Transaction(Database & db)
: txn(0)
{
begin(db);
}
Transaction::~Transaction()
{
if (txn) abort();
}
void Transaction::begin(Database & db)
{
assert(txn == 0);
db.requireEnv();
try {
db.env->txn_begin(0, &txn, 0);
} catch (DbException e) { rethrow(e); }
}
void Transaction::commit()
{
if (!txn) throw Error("commit called on null transaction");
debug(format("committing transaction %1%") % (void *) txn);
DbTxn * txn2 = txn;
txn = 0;
try {
txn2->commit(0);
} catch (DbException e) { rethrow(e); }
}
void Transaction::abort()
{
if (!txn) throw Error("abort called on null transaction");
debug(format("aborting transaction %1%") % (void *) txn);
DbTxn * txn2 = txn;
txn = 0;
try {
txn2->abort();
} catch (DbException e) { rethrow(e); }
}
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void Transaction::moveTo(Transaction & t)
{
if (t.txn) throw Error("target txn already exists");
t.txn = txn;
txn = 0;
}
void Database::requireEnv()
{
checkInterrupt();
if (!env) throw Error("database environment is not open "
"(maybe you don't have sufficient permission?)");
}
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Db * Database::getDb(TableId table)
{
if (table == 0)
throw Error("database table is not open "
"(maybe you don't have sufficient permission?)");
map<TableId, Db *>::iterator i = tables.find(table);
if (i == tables.end())
throw Error("unknown table id");
return i->second;
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}
Database::Database()
: env(0)
, nextId(1)
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{
}
Database::~Database()
{
close();
}
int getAccessorCount(int fd)
{
if (lseek(fd, 0, SEEK_SET) == -1)
throw SysError("seeking accessor count");
char buf[128];
int len;
if ((len = read(fd, buf, sizeof(buf) - 1)) == -1)
throw SysError("reading accessor count");
buf[len] = 0;
int count;
if (sscanf(buf, "%d", &count) != 1) {
debug(format("accessor count is invalid: `%1%'") % buf);
return -1;
}
return count;
}
void setAccessorCount(int fd, int n)
{
if (lseek(fd, 0, SEEK_SET) == -1)
throw SysError("seeking accessor count");
string s = (format("%1%") % n).str();
const char * s2 = s.c_str();
if (write(fd, s2, strlen(s2)) != (ssize_t) strlen(s2) ||
ftruncate(fd, strlen(s2)) != 0)
throw SysError("writing accessor count");
}
void openEnv(DbEnv * env, const string & path, u_int32_t flags)
{
env->open(path.c_str(),
DB_INIT_LOCK | DB_INIT_LOG | DB_INIT_MPOOL | DB_INIT_TXN |
DB_CREATE | flags,
0666);
}
static int my_fsync(int fd)
{
return 0;
}
void Database::open(const string & path)
{
if (env) throw Error(format("environment already open"));
try {
debug(format("opening database environment"));
/* Create the database environment object. */
DbEnv * env = 0; /* !!! close on error */
env = new DbEnv(0);
/* Smaller log files. */
env->set_lg_bsize(32 * 1024); /* default */
env->set_lg_max(256 * 1024); /* must be > 4 * lg_bsize */
/* Write the log, but don't sync. This protects transactions
against application crashes, but if the system crashes,
some transactions may be undone. An acceptable risk, I
think. */
env->set_flags(DB_TXN_WRITE_NOSYNC | DB_LOG_AUTOREMOVE, 1);
/* Increase the locking limits. If you ever get `Dbc::get:
Cannot allocate memory' or similar, especially while
running `nix-store --verify', just increase the following
number, then run db_recover on the database to remove the
existing DB environment (since changes only take effect on
new environments). */
env->set_lk_max_locks(4000);
env->set_lk_max_lockers(4000);
env->set_lk_max_objects(4000);
env->set_lk_detect(DB_LOCK_DEFAULT);
/* Dangerous, probably, but from the docs it *seems* that BDB
shouldn't sync when DB_TXN_WRITE_NOSYNC is used, but it
still fsync()s sometimes. */
db_env_set_func_fsync(my_fsync);
/* The following code provides automatic recovery of the
database environment. Recovery is necessary when a process
dies while it has the database open. To detect this,
processes atomically increment a counter when they open the
database, and decrement it when they close it. If we see
that counter is > 0 but no processes are accessing the
database---determined by attempting to obtain a write lock
on a lock file on which all accessors have a read lock---we
must run recovery. Note that this also ensures that we
only run recovery when there are no other accessors (which
could cause database corruption). */
/* !!! close fdAccessors / fdLock on exception */
/* Open the accessor count file. */
string accessorsPath = path + "/accessor_count";
fdAccessors = ::open(accessorsPath.c_str(), O_RDWR | O_CREAT, 0666);
if (fdAccessors == -1)
if (errno == EACCES)
throw DbNoPermission(
format("permission denied to database in `%1%'") % accessorsPath);
else
throw SysError(format("opening file `%1%'") % accessorsPath);
/* Open the lock file. */
string lockPath = path + "/access_lock";
fdLock = ::open(lockPath.c_str(), O_RDWR | O_CREAT, 0666);
if (fdLock == -1)
throw SysError(format("opening lock file `%1%'") % lockPath);
/* Try to acquire a write lock. */
debug(format("attempting write lock on `%1%'") % lockPath);
if (lockFile(fdLock, ltWrite, false)) { /* don't wait */
debug(format("write lock granted"));
/* We have a write lock, which means that there are no
other readers or writers. */
int n = getAccessorCount(fdAccessors);
if (n != 0) {
printMsg(lvlTalkative,
format("accessor count is %1%, running recovery") % n);
/* Open the environment after running recovery. */
openEnv(env, path, DB_RECOVER);
}
else
/* Open the environment normally. */
openEnv(env, path, 0);
setAccessorCount(fdAccessors, 1);
/* Downgrade to a read lock. */
debug(format("downgrading to read lock on `%1%'") % lockPath);
lockFile(fdLock, ltRead, true);
} else {
/* There are other accessors. */
debug(format("write lock refused"));
/* Acquire a read lock. */
debug(format("acquiring read lock on `%1%'") % lockPath);
lockFile(fdLock, ltRead, true); /* wait indefinitely */
/* Increment the accessor count. */
lockFile(fdAccessors, ltWrite, true);
int n = getAccessorCount(fdAccessors) + 1;
setAccessorCount(fdAccessors, n);
debug(format("incremented accessor count to %1%") % n);
lockFile(fdAccessors, ltNone, true);
/* Open the environment normally. */
openEnv(env, path, 0);
}
this->env = env;
} catch (DbException e) { rethrow(e); }
}
void Database::close()
{
if (!env) return;
/* Close the database environment. */
debug(format("closing database environment"));
try {
for (map<TableId, Db *>::iterator i = tables.begin();
i != tables.end(); i++)
{
Db * db = i->second;
db->close(DB_NOSYNC);
delete db;
}
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/* Do a checkpoint every 128 kilobytes, or every 5 minutes. */
env->txn_checkpoint(128, 5, 0);
env->close(0);
} catch (DbException e) { rethrow(e); }
delete env;
/* Decrement the accessor count. */
lockFile(fdAccessors, ltWrite, true);
int n = getAccessorCount(fdAccessors) - 1;
setAccessorCount(fdAccessors, n);
debug(format("decremented accessor count to %1%") % n);
lockFile(fdAccessors, ltNone, true);
::close(fdAccessors);
::close(fdLock);
}
TableId Database::openTable(const string & tableName)
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{
requireEnv();
TableId table = nextId++;
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try {
Db * db = new Db(env, 0);
try {
db->open(0, tableName.c_str(), 0,
DB_HASH, DB_CREATE | DB_AUTO_COMMIT, 0666);
} catch (...) {
delete db;
throw;
}
tables[table] = db;
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} catch (DbException e) { rethrow(e); }
return table;
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}
bool Database::queryString(const Transaction & txn, TableId table,
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const string & key, string & data)
{
checkInterrupt();
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try {
Db * db = getDb(table);
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Dbt kt((void *) key.c_str(), key.length());
Dbt dt;
int err = db->get(txn.txn, &kt, &dt, 0);
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if (err) return false;
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if (!dt.get_data())
data = "";
else
data = string((char *) dt.get_data(), dt.get_size());
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} catch (DbException e) { rethrow(e); }
return true;
}
bool Database::queryStrings(const Transaction & txn, TableId table,
const string & key, Strings & data)
{
string d;
if (!queryString(txn, table, key, d))
return false;
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data = unpackStrings(d);
return true;
}
void Database::setString(const Transaction & txn, TableId table,
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const string & key, const string & data)
{
checkInterrupt();
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try {
Db * db = getDb(table);
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Dbt kt((void *) key.c_str(), key.length());
Dbt dt((void *) data.c_str(), data.length());
db->put(txn.txn, &kt, &dt, 0);
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} catch (DbException e) { rethrow(e); }
}
void Database::setStrings(const Transaction & txn, TableId table,
const string & key, const Strings & data, bool deleteEmpty)
{
if (deleteEmpty && data.size() == 0)
delPair(txn, table, key);
else
setString(txn, table, key, packStrings(data));
}
void Database::delPair(const Transaction & txn, TableId table,
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const string & key)
{
checkInterrupt();
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try {
Db * db = getDb(table);
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Dbt kt((void *) key.c_str(), key.length());
db->del(txn.txn, &kt, 0);
/* Non-existence of a pair with the given key is not an
error. */
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} catch (DbException e) { rethrow(e); }
}
void Database::enumTable(const Transaction & txn, TableId table,
Strings & keys)
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{
try {
Db * db = getDb(table);
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Dbc * dbc;
db->cursor(txn.txn, &dbc, 0);
DestroyDbc destroyDbc(dbc);
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Dbt kt, dt;
while (dbc->get(&kt, &dt, DB_NEXT) != DB_NOTFOUND) {
checkInterrupt();
keys.push_back(
string((char *) kt.get_data(), kt.get_size()));
}
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} catch (DbException e) { rethrow(e); }
}