ENV: allow a user-configurable number of make jobs

Let an environment variable, HOMEBREW_MAKE_JOBS, override the default
'-j<cores>' make flag.

Now we can more easily debug formula that normally build in parallel, or
(potentially) speed up lengthy builds.

2 files changed, 16 insertions, 1 deletions

diff --git a/Library/Contributions/manpages/brew.1.md b/Library/Contributions/manpages/brew.1.md
index 89d73f612..533937284 100644
--- a/Library/Contributions/manpages/brew.1.md
+++ b/Library/Contributions/manpages/brew.1.md
@@ -316,6 +316,12 @@ Some sample commands ship with Homebrew and are enabled by default.
     If set, Homebrew will not remove files from `share/info`, allowing them
     to be linked from the Cellar.
 
+  * HOMEBREW\_MAKE\_JOBS:
+    If set, instructs Homebrew to use the value of `HOMEBREW_MAKE_JOBS` as
+    the number of parallel jobs to run when building with `make`(1).
+
+    *Default:* the number of available CPU cores.
+
   * HOMEBREW\_SVN:
     When exporting from Subversion, Homebrew will use `HOMEBREW_SVN` if set,
     a Homebrew-built Subversion if installed, or the system-provided binary.
diff --git a/Library/Homebrew/extend/ENV.rb b/Library/Homebrew/extend/ENV.rb
index 931f69271..8ca166ac0 100644
--- a/Library/Homebrew/extend/ENV.rb
+++ b/Library/Homebrew/extend/ENV.rb
@@ -8,7 +8,7 @@ module HomebrewEnvExtension
     delete('CPPFLAGS')
     delete('LDFLAGS')
 
-    self['MAKEFLAGS'] = "-j#{Hardware.processor_count}"
+    self['MAKEFLAGS'] = "-j#{self.make_jobs}"
 
     unless HOMEBREW_PREFIX.to_s == '/usr/local'
       # /usr/local is already an -isystem and -L directory so we skip it
@@ -309,4 +309,13 @@ Please take one of the following actions:
   def use_llvm?
     self['HOMEBREW_USE_LLVM'] or ARGV.include? '--use-llvm'
   end
+
+  def make_jobs
+    # '-j' requires a positive integral argument
+    if self['HOMEBREW_MAKE_JOBS'].to_i > 0
+      self['HOMEBREW_MAKE_JOBS']
+    else
+      Hardware.processor_count
+    end
+  end
 end