Prevent slowdown in yum due to high file descriptor count

Set the maximum number of file descriptors to 1024 before going through the `yum
install` step of the build, only in case `yum_requirements.txt` is present in
the recipe. This is done to mitigate a bug with old versions of rpm such as the
one shipped with the Centos7 container. See https://bugzilla.redhat.com/show_bug.cgi?id=1537564

conda_smithy/templates/build_steps.sh.tmpl

@@ -62,7 +62,12 @@ setup_conda_rc "${FEEDSTOCK_ROOT}" "${RECIPE_ROOT}" "${CONFIG_FILE}"
 {% if build_setup -%}
 {{ build_setup }}{% endif -%}
 {% if yum_build_setup is defined -%}
-{{ yum_build_setup }}{% endif -%}
+# Due to https://bugzilla.redhat.com/show_bug.cgi?id=1537564 old versions of rpm
+# are drastically slowed down when the number of file descriptors is very high.
+# This can be visible during a `yum install` step of a feedstock build.
+ulimit -n 1024
+{{ yum_build_setup }}
+{% endif -%}
 
 # make the build number clobber
 make_build_number "${FEEDSTOCK_ROOT}" "${RECIPE_ROOT}" "${CONFIG_FILE}"

news/1958_yum_fix_ulimit.rst

@@ -0,0 +1,26 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* Changed the build_steps.sh template so that it sets the number of maximum file
+  descriptors to 1024. This is done to mitigate a bug in old rpm versions (such
+  as the one shipped with the Centos7 container) that cause the yum install step
+  to take tremendously longer than necessary. See https://bugzilla.redhat.com/show_bug.cgi?id=1537564
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>