Reduce risk of incomplete JSON; fix Vim crash

When editing a file with quick-lint-js' Vim plugin, quick-lint-js might
produce so many errors that it flushes stdout in the middle of writing a
message. If a crash soon follows, stdout isn't flushed, so the client
sees invalid JSON which is difficult to successfully parse. See this
strace snippet for example:

    write(1, "{\"qflist\": [{\"col\": 5, \"lnum\": 10, \"end_col\": 14, \"end_lnum\": 10, \"type\": \"E\", \"nr\": \"E033\", \"vcol\": 0, \"text\": \"redeclaration of global variable\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 5, \"lnum\": 11, \"end_col\": 13, \"end_lnum\": 11, \"type\": \"E\", \"nr\": \"E033\", \"vcol\": 0, \"text\": \"redeclaration of global variable\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 22, \"lnum\": 120, \"end_col\": 22, \"end_lnum\": 120, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 27, \"lnum\": 120, \"end_col\": 27, \"end_lnum\": 120, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 34, \"lnum\": 120, \"end_col\": 34, \"end_lnum\": 120, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 45, \"lnum\": 120, \"end_col\": 45, \"end_lnum\": 120, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 22, \"lnum\": 133, \"end_col\": 22, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 27, \"lnum\": 133, \"end_col\": 27, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 45, \"lnum\": 133, \"end_col\": 45, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 56, \"lnum\": 133, \"end_col\": 56, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E043\", \"vcol\": 0, \"text\": \"unexpected '\\\\' in identifier\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 59, \"lnum\": 133, \"end_col\": 59, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 25, \"lnum\": 147, \"end_col\": 25, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 26, \"lnum\": 147, \"end_col\": 26, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 40, \"lnum\": 147, \"end_col\": 40, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E054\", \"vcol\": 0, \"text\": \"unexpected token\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 53, \"lnum\": 147, \"end_col\": 53, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 58, \"lnum\": 147, \"end_col\": 58, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 64, \"lnum\": 147, \"end_col\": 118, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E040\", \"vcol\": 0, \"text\": \"unclosed string literal\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 64, \"lnum\": 147, \"end_col\": 64, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 64, \"lnum\": 147, \"end_col\": 118, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E040\", \"vcol\": 0, \"text\": \"unclosed string literal\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-documentation.mjs\"},\n{\"col\": 4, \"lnum\": 148, \"end_col\": 4, \"end_lnum\": 148, \"type\": \"E\", \"nr\": \"E025\", \"vcol\": 0, \"text\": \"missing comma between object literal entries\", \"bufnr\": 1, \"filename\": \"/tmp/vyQeHSV/1/error-doc", 4096) = 4096
    write(2, "../src/./quick-lint-js/parse.h:3248: internal check failed in visit_binding_element: false\n", 91) = 91
    --- SIGILL {si_signo=SIGILL, si_code=ILL_ILLOPN, si_addr=0x43004a} ---
    +++ killed by SIGILL (core dumped) +++

Reduce the chance of incomplete messages by flushing after writing each
message. This is what text_error_reporter does implicitly (because
writing '\n' to stderr flushes, at least on my machine):

    write(1, "{\"qflist\": [{\"col\": 5, \"lnum\": 10, \"end_col\": 14, \"end_lnum\": 10, \"type\": \"E\", \"nr\": \"E033\", \"vcol\": 0, \"text\": \"redeclaration of global variable\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 213) = 213
    write(1, ",\n{\"col\": 5, \"lnum\": 11, \"end_col\": 13, \"end_lnum\": 11, \"type\": \"E\", \"nr\": \"E033\", \"vcol\": 0, \"text\": \"redeclaration of global variable\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 203) = 203
    write(1, ",\n{\"col\": 22, \"lnum\": 120, \"end_col\": 22, \"end_lnum\": 120, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 27, \"lnum\": 120, \"end_col\": 27, \"end_lnum\": 120, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 34, \"lnum\": 120, \"end_col\": 34, \"end_lnum\": 120, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 45, \"lnum\": 120, \"end_col\": 45, \"end_lnum\": 120, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 22, \"lnum\": 133, \"end_col\": 22, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 27, \"lnum\": 133, \"end_col\": 27, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 45, \"lnum\": 133, \"end_col\": 45, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 56, \"lnum\": 133, \"end_col\": 56, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E043\", \"vcol\": 0, \"text\": \"unexpected '\\\\' in identifier\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 203) = 203
    write(1, ",\n{\"col\": 59, \"lnum\": 133, \"end_col\": 59, \"end_lnum\": 133, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 25, \"lnum\": 147, \"end_col\": 25, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 26, \"lnum\": 147, \"end_col\": 26, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 40, \"lnum\": 147, \"end_col\": 40, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E054\", \"vcol\": 0, \"text\": \"unexpected token\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 190) = 190
    write(1, ",\n{\"col\": 53, \"lnum\": 147, \"end_col\": 53, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 58, \"lnum\": 147, \"end_col\": 58, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 64, \"lnum\": 147, \"end_col\": 118, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E040\", \"vcol\": 0, \"text\": \"unclosed string literal\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 198) = 198
    write(1, ",\n{\"col\": 64, \"lnum\": 147, \"end_col\": 64, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E027\", \"vcol\": 0, \"text\": \"missing semicolon after statement\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 207) = 207
    write(1, ",\n{\"col\": 64, \"lnum\": 147, \"end_col\": 118, \"end_lnum\": 147, \"type\": \"E\", \"nr\": \"E040\", \"vcol\": 0, \"text\": \"unclosed string literal\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 198) = 198
    write(1, ",\n{\"col\": 4, \"lnum\": 148, \"end_col\": 4, \"end_lnum\": 148, \"type\": \"E\", \"nr\": \"E025\", \"vcol\": 0, \"text\": \"missing comma between object literal entries\", \"bufnr\": 1, \"filename\": \"/tmp/vseTukR/1/error-documentation.mjs\"}", 216) = 216
    write(2, "../src/./quick-lint-js/parse.h:3248: internal check failed in visit_binding_element: false\n", 91) = 91
    --- SIGILL {si_signo=SIGILL, si_code=ILL_ILLOPN, si_addr=0x4300aa} ---
    +++ killed by SIGILL (core dumped) +++

This fixes our Vim plugin complaining about invalid JSON.

I considered making QLJS_ASSERT flush stdout, but some crashes (e.g.
segfaults) would still cause problems.

Author: strager

src/vim-qflist-json-error-reporter.cpp

@@ -135,6 +135,17 @@ void vim_qflist_json_error_formatter::write_after_message(
     this->output_ << '"';
   }
   this->output_ << '}';
+
+  // If we don't flush, output is buffered. A lot of messages (e.g. over 4 KiB)
+  // will fill the buffer and possibly force a flush in the middle of a message.
+  // Then, a crash in the future (e.g. an assertion failure) will leave an
+  // incomplete message written to the client. The client will have a hard time
+  // extracting partial information from the incomplete JSON.
+  //
+  // If we flush now, it's less likely that a message ends up on the client. The
+  // client can easily recover by adding a '}' at the end of the input to make
+  // the incomplete JSON valid.
+  this->output_.flush();
 }
 }