aboutsummaryrefslogtreecommitdiff
path: root/build2/depdb.cxx
blob: c74536cf10910ebb393596c60bb59679b18bf132 (plain)
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
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
// file      : build2/depdb.cxx -*- C++ -*-
// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
// license   : MIT; see accompanying LICENSE file

#include <build2/depdb.hxx>

#include <libbutl/filesystem.hxx> // file_mtime()

using namespace std;
using namespace butl;

namespace build2
{
  depdb::
  depdb (const path& f)
      : mtime_ (file_mtime (f)), touch_ (false)
  {
    fs_.exceptions (fstream::failbit | fstream::badbit);

    if (mtime_ != timestamp_nonexistent)
    {
      // Open an existing file.
      //
      fs_.open (f.string (), fstream::in | fstream::out | fstream::binary);
      state_ = state::read;
      fs_.exceptions (fstream::badbit);

      // Read the database format version.
      //
      string* l (read ());
      if (l == nullptr || *l != "1")
        write ('1');
    }
    else
    {
      fs_.open (f.string (), fstream::out | fstream::binary);

      state_ = state::write;
      mtime_ = timestamp_unknown;

      write ('1');
    }
  }

  void depdb::
  change (bool flush)
  {
    assert (state_ != state::write);

    fs_.clear ();
    fs_.exceptions (fstream::failbit | fstream::badbit);

    // Consider this scenario: we are overwriting an old line (so it ends with
    // a newline and the "end marker") but the operation failed half way
    // through. Now we have the prefix from the new line, the suffix from the
    // old, and everything looks valid. So what we need is to somehow
    // invalidate the old content so that it can never combine with (partial)
    // new content to form a valid line. One way would be to truncate the file
    // but that is not straightforward (see note in close()). Alternatively,
    // we can replace everything with the "end markers".
    //
    fs_.seekg (0, fstream::end);
    fstream::pos_type end (fs_.tellg ());

    if (end != pos_)
    {
      fs_.seekp (pos_);

      for (auto i (end - pos_); i != 0; --i)
        fs_.put ('\0');

      if (flush)
        fs_.flush ();
    }

    fs_.seekp (pos_); // Must be done when changing from read to write.

    state_ = state::write;
    mtime_ = timestamp_unknown;
  }

  string* depdb::
  read_ ()
  {
    // Save the start position of this line so that we can overwrite it.
    //
    pos_ = fs_.tellg ();

    // Note that we intentionally check for eof after updating the write
    // position.
    //
    if (state_ == state::read_eof)
      return nullptr;

    getline (fs_, line_); // Calls data_.erase().

    // The line should always end with a newline. If it doesn't, then this
    // line (and the rest of the database) is assumed corrupted. Also peek at
    // the character after the newline. We should either have the next line or
    // '\0', which is our "end marker", that is, it indicates the database
    // was properly closed.
    //
    fstream::int_type c;
    if (fs_.fail () || // Nothing got extracted.
        fs_.eof ()  || // Eof reached before delimiter.
        (c = fs_.peek ()) == fstream::traits_type::eof ())
    {
      // Preemptively switch to writing. While we could have delayed this
      // until the user called write(), if the user calls read() again (for
      // whatever misguided reason) we will mess up the overwrite position.
      //
      change ();
      return nullptr;
    }

    // Handle the "end marker". Note that the caller can still switch to the
    // write mode on this line. And, after calling read() again, write to the
    // next line (i.e., start from the "end marker").
    //
    if (c == '\0')
      state_ = state::read_eof;

    return &line_;
  }

  bool depdb::
  skip ()
  {
    if (state_ == state::read_eof)
      return true;

    assert (state_ == state::read);

    // The rest is pretty similar in logic to read_() above.
    //
    pos_ = fs_.tellg ();

    // Keep reading lines checking for the end marker after each newline.
    //
    fstream::int_type c;
    do
    {
      if ((c = fs_.get ()) == '\n')
      {
        if ((c = fs_.get ()) == '\0')
        {
          state_ = state::read_eof;
          return true;
        }
      }
    } while (c != fstream::traits_type::eof ());

    // Invalid database so change over to writing.
    //
    change ();
    return false;
  }

  void depdb::
  write (const char* s, size_t n, bool nl)
  {
    // Switch to writing if we are still reading.
    //
    if (state_ != state::write)
      change ();

    fs_.write (s, static_cast<streamsize> (n));

    if (nl)
      fs_.put ('\n');
  }

  void depdb::
  write (char c, bool nl)
  {
    // Switch to writing if we are still reading.
    //
    if (state_ != state::write)
      change ();

    fs_.put (c);

    if (nl)
      fs_.put ('\n');
  }

  void depdb::
  close ()
  {
    // If we are at eof, then it means all lines are good, there is the "end
    // marker" at the end, and we don't need to do anything, except, maybe
    // touch the file. Otherwise, we need to add the "end marker" and truncate
    // the rest.
    //
    if (state_ == state::read_eof)
    {
      // While there are utime(2)/utimensat(2) (and probably something similar
      // for Windows), for now we just overwrite the "end marker". Hopefully
      // no implementation will be smart enough to recognize this is a no-op
      // and skip updating mtime (which would probably be incorrect).
      //
      // It would be interesting to one day write an implementation that uses
      // POSIX file OI, futimens(), and ftruncate() and see how much better it
      // performs.
      //
      if (touch_)
      {
        fs_.clear ();
        fs_.exceptions (fstream::failbit | fstream::badbit);
        fs_.seekp (0, fstream::cur); // Required to switch from read to write.
        fs_.put ('\0');
      }
    }
    else
    {
      if (state_ != state::write)
      {
        pos_ = fs_.tellg (); // The last line is accepted.
        change (false); // Don't flush.
      }

      fs_.put ('\0'); // The "end marker".

      // Truncating an fstream is actually a non-portable pain in the butt.
      // What if we leave the junk after the "end marker"? These files are
      // pretty small and chances are they will occupy the filesystem's block
      // size (usually 4KB) whether they are truncated or not. So it might
      // actually be faster not to truncate.
    }

    fs_.close ();
  }
}