Create a gn link pool which is disabled by default.

The oss-fuzz build of Skia seems to have difficulty when it does too
many linker steps at a time. Ninja will generally not launch new build
steps if the load is too high, but the oss-fuzz builder machines are
slow enough that the load doesn't seem to rise fast enough to prevent
too many linkers from be started at a time.

By default this change does not request a linker pool at all and the
user must request one by setting the link_pool_depth. Negative values
disable the linker pool, zero sets the number to the number of cpus, and
positive values set the pool size. This should allow for some
experimentation while testing what pool size is appropriate for the fuzz
builders while not changing current default behavior.

Bug: oss-fuzz:23438,oss-fuzz:24345
Change-Id: Iba62adac3bea4d968441aa08e33918b5ae962c7b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306646
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

Author: Ben Wagner

gn/toolchain/BUILD.gn

@@ -40,6 +40,11 @@ declare_args() {
   # slowest steps in a build, so we don't want to limit too much. Use the number
   # of CPUs as a default.
   dlsymutil_pool_depth = exec_script("num_cpus.py", [], "value")
+
+  # Too many linkers running at once causes issues for some builders. Allow
+  # such builders to limit the number of concurrent link steps.
+  # link_pool_depth < 0 means no pool, 0 means cpu count, > 0 sets pool size.
+  link_pool_depth = -1
 }
 
 # For 'shell' see https://ninja-build.org/manual.html#ref_rule_command
@@ -55,6 +60,15 @@ if (current_toolchain == default_toolchain) {
   pool("dsymutil_pool") {
     depth = dlsymutil_pool_depth
   }
+  if (0 <= link_pool_depth) {
+    pool("link_pool") {
+      if (link_pool_depth == 0) {
+        depth = exec_script("num_cpus.py", [], "value")
+      } else {
+        depth = link_pool_depth
+      }
+    }
+  }
 }
 
 toolchain("msvc") {
@@ -147,6 +161,9 @@ toolchain("msvc") {
     # inputs_newline works around a fixed per-line buffer size in the linker.
     rspfile_content = "{{inputs_newline}}"
     description = "link {{output}}"
+    if (0 <= link_pool_depth) {
+      pool = ":link_pool($default_toolchain)"
+    }
   }
 
   tool("solink") {
@@ -177,6 +194,9 @@ toolchain("msvc") {
     # inputs_newline works around a fixed per-line buffer size in the linker.
     rspfile_content = "{{inputs_newline}} {{libs}} {{solibs}} {{ldflags}}"
     description = "link {{output}}"
+    if (0 <= link_pool_depth) {
+      pool = ":link_pool($default_toolchain)"
+    }
   }
 
   tool("link") {
@@ -192,6 +212,9 @@ toolchain("msvc") {
     # inputs_newline works around a fixed per-line buffer size in the linker.
     rspfile_content = "{{inputs_newline}} {{libs}} {{solibs}} {{ldflags}}"
     description = "link {{output}}"
+    if (0 <= link_pool_depth) {
+      pool = ":link_pool($default_toolchain)"
+    }
   }
 
   tool("stamp") {
@@ -278,6 +301,9 @@ template("gcc_like_toolchain") {
       default_output_extension = ".a"
       output_prefix = "lib"
       description = "link {{output}}"
+      if (0 <= link_pool_depth) {
+        pool = ":link_pool($default_toolchain)"
+      }
     }
 
     tool("solink") {
@@ -309,6 +335,9 @@ template("gcc_like_toolchain") {
       output_prefix = "lib"
       default_output_extension = ".so"
       description = "link {{output}}"
+      if (0 <= link_pool_depth) {
+        pool = ":link_pool($default_toolchain)"
+      }
     }
 
     tool("link") {
@@ -332,6 +361,9 @@ template("gcc_like_toolchain") {
 
       outputs = [ "$exe_name" ]
       description = "link {{output}}"
+      if (0 <= link_pool_depth) {
+        pool = ":link_pool($default_toolchain)"
+      }
     }
 
     tool("stamp") {