Add LIBXML_NOBLANKS to dom_load() XML parse options

[jordikroon]

Drops whitespace-only text nodes. Which shouldn't be harmful. It results in a smaller DOM, thus faster to traverse and requires less memory.

Benchmarked configure.php --with-lang=en, 5 runs, PHP 8.5:

for i in 1 2 3 4 5; do
  /usr/bin/time -l php configure.php --with-lang=en >/dev/null
done 2>benchmark.txt

Benchmark:

	Before	After
Time	8.94s	6.28s
Peak memory	1687 MB	1263 MB

Raw files:

Before patch (baseline)

=== baseline run 1 ===
        8.94 real         5.79 user         2.19 sys
          1703395328  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              178083  page reclaims
                2166  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
               23136  voluntary context switches
               10176  involuntary context switches
         49464100831  instructions retired
         15879556244  cycles elapsed
          1687130560  peak memory footprint
=== baseline run 2 ===
        8.36 real         5.91 user         3.10 sys
          1745633280  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              137481  page reclaims
                 410  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                2017  voluntary context switches
               11477  involuntary context switches
         50933601605  instructions retired
         18544221387  cycles elapsed
          1687310784  peak memory footprint
=== baseline run 3 ===
        9.61 real         6.00 user         3.05 sys
          1650540544  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              226507  page reclaims
                2176  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
               13407  voluntary context switches
               17884  involuntary context switches
         50502793793  instructions retired
         16581835963  cycles elapsed
          1687376320  peak memory footprint
=== baseline run 4 ===
        6.83 real         5.81 user         1.75 sys
          1761476608  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              137900  page reclaims
                 391  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                 271  voluntary context switches
                6098  involuntary context switches
         48201588467  instructions retired
         14541027505  cycles elapsed
          1687343552  peak memory footprint
=== baseline run 5 ===
       10.23 real         6.30 user         3.08 sys
          1748795392  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              138801  page reclaims
                2065  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
               15892  voluntary context switches
               25486  involuntary context switches
         51353051988  instructions retired
         18729227016  cycles elapsed
          1687310784  peak memory footprint

After patch

=== patched run 1 ===
        6.40 real         5.35 user         1.83 sys
          1306656768  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              111176  page reclaims
                 486  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                 967  voluntary context switches
                7422  involuntary context switches
         42787245602  instructions retired
         13223610026  cycles elapsed
          1263193728  peak memory footprint
=== patched run 2 ===
        5.64 real         5.08 user         1.57 sys
          1313062912  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              111184  page reclaims
                 371  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                 241  voluntary context switches
                3070  involuntary context switches
         42706120047  instructions retired
         12344963394  cycles elapsed
          1263554176  peak memory footprint
=== patched run 3 ===
        9.44 real         5.80 user         3.54 sys
          1300905984  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              111008  page reclaims
                1975  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                8799  voluntary context switches
               16634  involuntary context switches
         45579012902  instructions retired
         16727081777  cycles elapsed
          1263423104  peak memory footprint
=== patched run 4 ===
        6.28 real         5.24 user         1.75 sys
          1243627520  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              131634  page reclaims
                 477  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                1024  voluntary context switches
                8285  involuntary context switches
         42888761617  instructions retired
         13168907040  cycles elapsed
          1263324800  peak memory footprint
=== patched run 5 ===
        5.81 real         5.22 user         1.61 sys
          1309032448  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
              111131  page reclaims
                 396  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                 168  voluntary context switches
                3945  involuntary context switches
         42648784251  instructions retired
         12410583670  cycles elapsed
          1263062656  peak memory footprint

[alfsb]

As commented on discord, I know little to nothing about PhD in this respect.

Why PhD? Because some Docbook parts specify that whitespace should be preserved in the rendering process, so some whitespace may end up being relevant in PhD, or in final output.

To get better performance and reduce memory, removing XML comments may give a similar impact, and these are unspecified in Docbook.

[alfsb]

Philip O gives an example where this causes a problem. The rendering of ltrim() will be broken, because two consecutive <acronym> separated by space will be source transformed, and then rendered, as two consecutive <acronym> glued together.

The problem is more general. Whitespace in Docbook is complicated. In some elements it is completely irrelevant, and could be trimmed, but there are some other contexts, where whitespace should be coalesced (like HTML), and other contexts where it should be fully preserved.

There is a hint in old libxml docs:

The 2.x and later version will switch to the XML standard way and ignorableWhitespace() are only generated when running the parser in validating mode and when the current element doesn't allow CDATA or mixed content.

So... it may be possible to change libxml to the "correct" Docbook behaviour if it is called in validating mode.

[alfsb]

After some reading, I think that LIBXML_NOBLANKS may only work with DTD schemas. That is because, Docbook is officially defined as RelaxNG, and

RELAX NG does not define a way for an XML document to associate itself with a RELAX NG pattern.

so DOMDocument->validateOnParse will only work with schemas that do that, and libxml2 has validateOnParse support. Namely, DTD schemas. @ndossche may correct me here.

The last time Docbook published a DTD schema was on version 5.0.1 AFAIK. And I do not know a way to convert a RNG into a DTD, even imperfectly. And obviously, enabling validateOnParse against a full Docbook specification may cause performance regression.

Maybe it is possible to just fake it. To bruteforce a "one rule per element" DTD schema that contains all pairwise parent vs child-element-or-text pairs. The theory is that libxml only needs to know if a particular element may contain #PCDATA, so LIBXML_NOBLANKS will not be active in these elements.

[alfsb]

This cannot be merged as is. As commented above, this changes the rendering of the manual in broken ways.

It may be possible, though. It is only a conjecture, but by creating a simplified DTD from RNGs (preferably) or from a validated manual (for experimental use only), with the format:

<!ELEMENT parent-element (child1|child2|child3)*>
<!ELEMENT parent-element (#PCDATA|child1|child2)*>

and enabling it on doc-base/manual.xml, may cause libxml2 to trim spaces only they not cause rendering changes.

The presence or absence of #PCDATA should be calculated by detecting if any of their possible direct child nodes is expected text nodes (from RNG) or is a text node not composed entirely of whitespace (from build XML).