Handle non-gnu libc on linux platforms in RubyGems

Attempting to install a gem published as both *-linux and *-linux-musl
results in the incorrect gem being picked up, causing build failures due
to binary incompatibility. This is caused by the `nil` wildcard
swallowing the libc information upon version comparison.

Handle the linux case by performing only non-wildcard equality on the
version and asserting 'gnu' and nil equivalence, while preserving the
current behaviour for other OSes.

Co-authored-by: Loic Nageleisen <loic.nageleisen@gmail.com>

lib/rubygems/platform.rb

@@ -151,10 +151,17 @@ def hash # :nodoc:
   ##
   # Does +other+ match this platform?  Two platforms match if they have the
   # same CPU, or either has a CPU of 'universal', they have the same OS, and
-  # they have the same version, or either has no version.
+  # they have the same version, or either one has no version
   #
   # Additionally, the platform will match if the local CPU is 'arm' and the
   # other CPU starts with "arm" (for generic ARM family support).
+  #
+  # Of note, this method is not commutative. Indeed the OS 'linux' has a
+  # special case: the version is the libc name, yet while "no version" stands
+  # as a wildcard for a binary gem platform (as for other OSes), for the
+  # runtime platform "no version" stands for 'gnu'. To be able to disinguish
+  # these, the method receiver is the gem platform, while the argument is
+  # the runtime platform.
 
   def ===(other)
     return nil unless Gem::Platform === other
@@ -171,7 +178,11 @@ def ===(other)
       @os == other.os &&
 
       # version
-      (@version.nil? || other.version.nil? || @version == other.version)
+      (
+        (@os != "linux" && (@version.nil? || other.version.nil?)) ||
+        (@os == "linux" && (@version.nil? && !other.version.nil?)) ||
+        @version == other.version
+      )
   end
 
   ##

test/rubygems/test_gem_platform.rb

@@ -135,7 +135,9 @@ def test_initialize
       "i386-solaris2.8"        => ["x86",       "solaris",   "2.8"],
       "mswin32"                => ["x86",       "mswin32",   nil],
       "x86_64-linux"           => ["x86_64",    "linux",     nil],
+      "x86_64-linux-gnu"       => ["x86_64",    "linux",     nil],
       "x86_64-linux-musl"      => ["x86_64",    "linux",     "musl"],
+      "x86_64-linux-uclibc"    => ["x86_64",    "linux",     "uclibc"],
       "x86_64-openbsd3.9"      => ["x86_64",    "openbsd",   "3.9"],
       "x86_64-openbsd4.0"      => ["x86_64",    "openbsd",   "4.0"],
       "x86_64-openbsd"         => ["x86_64",    "openbsd",   nil],
@@ -263,6 +265,34 @@ def test_nil_cpu_arch_is_treated_as_universal
     assert((with_x86_arch === with_nil_arch), "x86 =~ nil")
   end
 
+  def test_nil_version_is_treated_as_any_version
+    x86_darwin_8 = Gem::Platform.new "i686-darwin8.0"
+    x86_darwin_nil = Gem::Platform.new "i686-darwin"
+
+    assert((x86_darwin_8 === x86_darwin_nil), "8.0 =~ nil")
+    assert((x86_darwin_nil === x86_darwin_8), "nil =~ 8.0")
+  end
+
+  def test_nil_version_is_stricter_for_linux_os
+    x86_linux = Gem::Platform.new "i686-linux"
+    x86_linux_gnu = Gem::Platform.new "i686-linux-gnu"
+    x86_linux_musl = Gem::Platform.new "i686-linux-musl"
+    x86_linux_uclibc = Gem::Platform.new "i686-linux-uclibc"
+
+    # a naked linux runtime is implicit gnu, as it represents the common glibc-linked runtime
+    assert(x86_linux === x86_linux_gnu, "linux =~ linux-gnu")
+    assert(x86_linux_gnu === x86_linux, "linux-gnu =~ linux")
+
+    # explicit libc differ
+    refute(x86_linux_uclibc === x86_linux_musl, "linux-uclibc =~ linux-musl")
+    refute(x86_linux_musl === x86_linux_uclibc, "linux-musl =~ linux-uclibc")
+
+    # musl host runtime accepts libc-generic or statically linked gems...
+    assert(x86_linux === x86_linux_musl, "linux =~ linux-musl")
+    # ...but implicit gnu runtime generally does not accept musl-specific gems
+    refute(x86_linux_musl === x86_linux, "linux-musl =~ linux")
+  end
+
   def test_equals3_cpu_arm
     arm   = Gem::Platform.new "arm-linux"
     armv5 = Gem::Platform.new "armv5-linux"