avoid ostream sentries per json string character

we don't have to create an ostream sentry object for every character of a JSON
string we write. format a bunch of characters and flush them to the stream all
at once instead.

this doesn't affect small numbers of string characters, but larger numbers of
total JSON string characters written gain a lot. at 1MB of total string written
we gain almost 30%, at 16MB it's almost a factor of 3x. large numbers of JSON
string characters do occur naturally in a nixos system evaluation to generate
documentation (though this is now somewhat mitigated by caching the largest part
of nixos option docs).

benchmarked with

  hyperfine 'nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) {e})"' --warmup 1 -L e 1,4,256,4096,65536

before:

  Benchmark 1: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 1)"
    Time (mean ± σ):      12.5 ms ±   0.2 ms    [User: 9.2 ms, System: 4.0 ms]
    Range (min … max):    11.9 ms …  13.1 ms    223 runs

  Benchmark 2: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 4)"
    Time (mean ± σ):      12.5 ms ±   0.2 ms    [User: 9.3 ms, System: 3.8 ms]
    Range (min … max):    11.9 ms …  13.2 ms    220 runs

  Benchmark 3: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 256)"
    Time (mean ± σ):      13.2 ms ±   0.3 ms    [User: 9.8 ms, System: 4.0 ms]
    Range (min … max):    12.6 ms …  14.3 ms    205 runs

  Benchmark 4: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 4096)"
    Time (mean ± σ):      24.0 ms ±   0.4 ms    [User: 19.4 ms, System: 5.2 ms]
    Range (min … max):    22.7 ms …  25.8 ms    119 runs

  Benchmark 5: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 65536)"
    Time (mean ± σ):     196.0 ms ±   3.7 ms    [User: 171.2 ms, System: 25.8 ms]
    Range (min … max):   190.6 ms … 201.5 ms    14 runs

after:

  Benchmark 1: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 1)"
    Time (mean ± σ):      12.4 ms ±   0.3 ms    [User: 9.1 ms, System: 4.0 ms]
    Range (min … max):    11.7 ms …  13.3 ms    204 runs

  Benchmark 2: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 4)"
    Time (mean ± σ):      12.4 ms ±   0.2 ms    [User: 9.2 ms, System: 3.9 ms]
    Range (min … max):    11.8 ms …  13.0 ms    214 runs

  Benchmark 3: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 256)"
    Time (mean ± σ):      12.6 ms ±   0.2 ms    [User: 9.5 ms, System: 3.8 ms]
    Range (min … max):    12.1 ms …  13.3 ms    209 runs

  Benchmark 4: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 4096)"
    Time (mean ± σ):      15.9 ms ±   0.2 ms    [User: 11.4 ms, System: 5.1 ms]
    Range (min … max):    15.2 ms …  16.4 ms    171 runs

  Benchmark 5: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 65536)"
    Time (mean ± σ):      69.0 ms ±   0.9 ms    [User: 44.3 ms, System: 25.3 ms]
    Range (min … max):    67.2 ms …  70.9 ms    42 runs

1 files changed, 32 insertions, 10 deletions

diff --git a/src/libutil/json.cc b/src/libutil/json.cc
index 01331947e..3a981376f 100644
--- a/src/libutil/json.cc
+++ b/src/libutil/json.cc
@@ -7,16 +7,38 @@ namespace nix {
 
 void toJSON(std::ostream & str, const char * start, const char * end)
 {
-    str << '"';
-    for (auto i = start; i != end; i++)
-        if (*i == '\"' || *i == '\\') str << '\\' << *i;
-        else if (*i == '\n') str << "\\n";
-        else if (*i == '\r') str << "\\r";
-        else if (*i == '\t') str << "\\t";
-        else if (*i >= 0 && *i < 32)
-            str << "\\u" << std::setfill('0') << std::setw(4) << std::hex << (uint16_t) *i << std::dec;
-        else str << *i;
-    str << '"';
+    constexpr size_t BUF_SIZE = 4096;
+    char buf[BUF_SIZE + 7]; // BUF_SIZE + largest single sequence of puts
+    size_t bufPos = 0;
+
+    const auto flush = [&] {
+        str.write(buf, bufPos);
+        bufPos = 0;
+    };
+    const auto put = [&] (char c) {
+        buf[bufPos++] = c;
+    };
+
+    put('"');
+    for (auto i = start; i != end; i++) {
+        if (bufPos >= BUF_SIZE) flush();
+        if (*i == '\"' || *i == '\\') { put('\\'); put(*i); }
+        else if (*i == '\n') { put('\\'); put('n'); }
+        else if (*i == '\r') { put('\\'); put('r'); }
+        else if (*i == '\t') { put('\\'); put('t'); }
+        else if (*i >= 0 && *i < 32) {
+            const char hex[17] = "0123456789abcdef";
+            put('\\');
+            put('u');
+            put(hex[(uint16_t(*i) >> 12) & 0xf]);
+            put(hex[(uint16_t(*i) >>  8) & 0xf]);
+            put(hex[(uint16_t(*i) >>  4) & 0xf]);
+            put(hex[(uint16_t(*i) >>  0) & 0xf]);
+        }
+        else put(*i);
+    }
+    put('"');
+    flush();
 }
 
 void toJSON(std::ostream & str, const char * s)