gdbsupport, gdb: add read_text_file_to_string, use it in linux_common_core_of_thread

I would like to add more code to nat/linux-osdata.c that reads an entire
file from /proc or /sys and processes it as a string afterwards.  I
would like to avoid duplicating the somewhat error-prone code that reads
an entire file to a buffer.  I think we should have a utility function
that does that.

Add read_file_to_string to gdbsupport/filestuff.{c,h}, and make
linux_common_core_of_thread use it.  I want to make the new function
return an std::string, and because strtok doesn't play well with
std::string (it requires a `char *`, std::string::c_str returns a `const
char *`), change linux_common_core_of_thread to use std::string methods
instead.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I1793fda72a82969c28b944a84acb953f74c9230a

2 files changed, 41 insertions, 0 deletions

diff --git a/gdbsupport/filestuff.cc b/gdbsupport/filestuff.cc
index 2dfae5a..cf5fb13b 100644
--- a/gdbsupport/filestuff.cc
+++ b/gdbsupport/filestuff.cc
@@ -501,3 +501,40 @@ mkdir_recursive (const char *dir)
       component_start = component_end;
     }
 }
+
+/* See gdbsupport/filestuff.h.  */
+
+gdb::optional<std::string>
+read_text_file_to_string (const char *path)
+{
+  gdb_file_up file = gdb_fopen_cloexec (path, "r");
+  if (file == nullptr)
+    return {};
+
+  std::string res;
+  for (;;)
+    {
+      std::string::size_type start_size = res.size ();
+      constexpr int chunk_size = 1024;
+
+      /* Resize to accomodate CHUNK_SIZE bytes.  */
+      res.resize (start_size + chunk_size);
+
+      int n = fread (&res[start_size], 1, chunk_size, file.get ());
+      if (n == chunk_size)
+	continue;
+
+      gdb_assert (n < chunk_size);
+
+      /* Less than CHUNK means EOF or error.  If it's an error, return
+         no value.  */
+      if (ferror (file.get ()))
+	return {};
+
+      /* Resize the string according to the data we read.  */
+      res.resize (start_size + n);
+      break;
+    }
+
+  return res;
+}
diff --git a/gdbsupport/filestuff.h b/gdbsupport/filestuff.h
index 4bc9249..3336290 100644
--- a/gdbsupport/filestuff.h
+++ b/gdbsupport/filestuff.h
@@ -129,4 +129,8 @@ extern bool is_regular_file (const char *name, int *errno_ptr);
 
 extern bool mkdir_recursive (const char *dir);
 
+/* Read the entire content of file PATH into an std::string.  */
+
+extern gdb::optional<std::string> read_text_file_to_string (const char *path);
+
 #endif /* COMMON_FILESTUFF_H */