1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
|
#pragma once
///@file
#include <functional>
#include <limits>
#include <list>
#include <memory>
#include <cassert>
#include "sync.hh"
#include "ref.hh"
namespace nix {
/**
* This template class implements a simple pool manager of resources
* of some type R, such as database connections. It is used as
* follows:
*
* class Connection { ... };
*
* Pool<Connection> pool;
*
* {
* auto conn(pool.get());
* conn->exec("select ...");
* }
*
* Here, the Connection object referenced by ‘conn’ is automatically
* returned to the pool when ‘conn’ goes out of scope.
*/
template <class R>
class Pool
{
public:
/**
* A function that produces new instances of R on demand.
*/
typedef std::function<ref<R>()> Factory;
/**
* A function that checks whether an instance of R is still
* usable. Unusable instances are removed from the pool.
*/
typedef std::function<bool(const ref<R> &)> Validator;
private:
Factory factory;
Validator validator;
struct State
{
size_t inUse = 0;
size_t max;
std::vector<ref<R>> idle;
};
Sync<State> state;
std::condition_variable wakeup;
public:
Pool(size_t max = std::numeric_limits<size_t>::max(),
const Factory & factory = []() { return make_ref<R>(); },
const Validator & validator = [](ref<R> r) { return true; })
: factory(factory)
, validator(validator)
{
auto state_(state.lock());
state_->max = max;
}
void incCapacity()
{
auto state_(state.lock());
state_->max++;
/* we could wakeup here, but this is only used when we're
* about to nest Pool usages, and we want to save the slot for
* the nested use if we can
*/
}
void decCapacity()
{
auto state_(state.lock());
state_->max--;
}
~Pool()
{
auto state_(state.lock());
assert(!state_->inUse);
state_->max = 0;
state_->idle.clear();
}
class Handle
{
private:
Pool & pool;
std::shared_ptr<R> r;
bool bad = false;
friend Pool;
Handle(Pool & pool, std::shared_ptr<R> r) : pool(pool), r(r) { }
public:
Handle(Handle && h) : pool(h.pool), r(h.r) { h.r.reset(); }
Handle(const Handle & l) = delete;
~Handle()
{
if (!r) return;
{
auto state_(pool.state.lock());
if (!bad)
state_->idle.push_back(ref<R>(r));
assert(state_->inUse);
state_->inUse--;
}
pool.wakeup.notify_one();
}
R * operator -> () { return &*r; }
R & operator * () { return *r; }
void markBad() { bad = true; }
};
Handle get()
{
{
auto state_(state.lock());
/* If we're over the maximum number of instance, we need
to wait until a slot becomes available. */
while (state_->idle.empty() && state_->inUse >= state_->max)
state_.wait(wakeup);
while (!state_->idle.empty()) {
auto p = state_->idle.back();
state_->idle.pop_back();
if (validator(p)) {
state_->inUse++;
return Handle(*this, p);
}
}
state_->inUse++;
}
/* We need to create a new instance. Because that might take a
while, we don't hold the lock in the meantime. */
try {
Handle h(*this, factory());
return h;
} catch (...) {
auto state_(state.lock());
state_->inUse--;
wakeup.notify_one();
throw;
}
}
size_t count()
{
auto state_(state.lock());
return state_->idle.size() + state_->inUse;
}
size_t capacity()
{
return state.lock()->max;
}
void flushBad()
{
auto state_(state.lock());
std::vector<ref<R>> left;
for (auto & p : state_->idle)
if (validator(p))
left.push_back(p);
std::swap(state_->idle, left);
}
};
}
|
