[Refactor] Helper for reading yaml header (#7552)

* helper for reading yaml header

* nocov

Author: MichaelChirico

R/fread.R

@@ -182,39 +182,8 @@ yaml=FALSE, tmpdir=tempdir(), tz="UTC")
     call_args = names(match.call())
     if (is.character(skip))
       warningf("Combining a search string as 'skip' and reading a YAML header may not work as expected -- currently, reading will proceed to search for 'skip' from the beginning of the file, NOT from the end of the metadata; please file an issue on GitHub if you'd like to see more intuitive behavior supported.")
-    # create connection to stream header lines from file:
-    #   https://stackoverflow.com/questions/9871307
-    f = base::file(input, 'r')
-    first_line = readLines(f, n=1L)
-    n_read = 1L
-    yaml_border_re = '^#?---'
-    if (!grepl(yaml_border_re, first_line)) {
-      close(f)
-      stopf(
-        'Encountered <%s%s> at the first unskipped line (%d), which does not constitute the start to a valid YAML header (expecting something matching regex "%s"); please check your input and try again.',
-        substr(first_line, 1L, 50L), if (nchar(first_line) > 50L) '...' else '', 1L+skip, yaml_border_re
-      )
-    }
-
-    yaml_comment_re = '^#'
-    yaml_string = character(0L)
-    repeat {
-      this_line = readLines(f, n=1L)
-      n_read = n_read + 1L
-      if (!length(this_line)){
-        close(f)
-        stopf('Reached the end of the file before finding a completion to the YAML header. A valid YAML header is bookended by lines matching the regex "%s". Please double check the input file is a valid csvy.', yaml_border_re)
-      }
-      if (grepl(yaml_border_re, this_line)) break
-      if (grepl(yaml_comment_re, this_line))
-        this_line = sub(yaml_comment_re, '', this_line)
-      yaml_string = paste(yaml_string, this_line, sep='\n')
-    }
-    close(f) # when #561 is implemented, no need to close f.
-
-    yaml_header = yaml::yaml.load(yaml_string)
+    yaml_header = .read_yaml_header(input, skip, verbose)
     yaml_names = names(yaml_header)
-    if (verbose) catf('Processed %d lines of YAML metadata with the following top-level fields: %s\n', n_read, brackify(yaml_names))
     # process header first since it impacts how to handle colClasses
     if ('header' %chin% yaml_names) {
       if ('header' %chin% call_args) messagef("User-supplied 'header' will override that found in metadata.")
@@ -384,6 +353,44 @@ yaml=FALSE, tmpdir=tempdir(), tz="UTC")
   ans
 }
 
+# nocov start. Covered only in other.Rraw
+.read_yaml_header = function(f, skip, verbose) {
+  # create connection to stream header lines from file:
+  #   https://stackoverflow.com/questions/9871307
+  con = base::file(f, 'r')
+  # NB: close() won't be right if 'f' itself can be a file connection (#561)
+  on.exit(close(con))
+
+  first_line = readLines(con, n=1L)
+  n_read = 1L
+  yaml_border_re = '^#?---'
+  if (!grepl(yaml_border_re, first_line)) {
+    stopf(
+      'Encountered <%s%s> at the first unskipped line (%d), which does not constitute the start to a valid YAML header (expecting something matching regex "%s"); please check your input and try again.',
+      substr(first_line, 1L, 50L), if (nchar(first_line) > 50L) '...' else '', 1L+skip, yaml_border_re
+    )
+  }
+
+  yaml_comment_re = '^#'
+  yaml_string = character(0L)
+  repeat {
+    this_line = readLines(con, n=1L)
+    n_read = n_read + 1L
+    if (!length(this_line)){
+      stopf('Reached the end of the file before finding a completion to the YAML header. A valid YAML header is bookended by lines matching the regex "%s". Please double check the input file is a valid csvy.', yaml_border_re)
+    }
+    if (grepl(yaml_border_re, this_line)) break
+    if (grepl(yaml_comment_re, this_line))
+      this_line = sub(yaml_comment_re, '', this_line)
+    yaml_string = paste(yaml_string, this_line, sep='\n')
+  }
+
+  yaml_header = yaml::yaml.load(yaml_string)
+  if (verbose) catf('Processed %d lines of YAML metadata with the following top-level fields: %s\n', n_read, brackify(names(yaml_header)))
+  yaml_header
+}
+# nocov end.
+
 known_signatures = list(
   zip = as.raw(c(0x50, 0x4b, 0x03, 0x04)), # charToRaw("PK\x03\x04")
   gzip = as.raw(c(0x1F, 0x8B)),