clean up relationship between Mime & Easy2 objects

 - The raw mime handle is now wrapped by a rust struct which will call curl_mime_free() upon Drop
 - Easy2 now keeps the MimeHandle as an Option instead of Vec
 - The above Option is kept inside Inner instead of directly under Easy2

src/easy/handler.rs

@@ -16,7 +16,7 @@ use socket2::Socket;
 use crate::easy::form;
 use crate::easy::list;
 #[cfg(feature = "mime")]
-use crate::easy::mime::Mime;
+use crate::easy::mime::{Mime, MimeHandle};
 use crate::easy::windows;
 use crate::easy::{Form, List};
 use crate::panic;
@@ -380,9 +380,6 @@ pub fn ssl_ctx(cx: *mut c_void) -> Result<(), Error> {
 /// ```
 pub struct Easy2<H> {
     inner: Box<Inner<H>>,
-    #[cfg(feature = "mime")]
-    /// Mime handles to free upon drop
-    mimes: Vec<*mut curl_sys::curl_mime>,
 }
 
 struct Inner<H> {
@@ -393,6 +390,9 @@ struct Inner<H> {
     form: Option<Form>,
     error_buf: RefCell<Vec<u8>>,
     handler: H,
+    #[cfg(feature = "mime")]
+    /// [MimeHandle] object to drop when it's safe
+    mime: Option<MimeHandle>,
 }
 
 unsafe impl<H: Send> Send for Inner<H> {}
@@ -603,9 +603,9 @@ impl<H: Handler> Easy2<H> {
                     form: None,
                     error_buf: RefCell::new(vec![0; curl_sys::CURL_ERROR_SIZE]),
                     handler,
+                    #[cfg(feature = "mime")]
+                    mime: Default::default(),
                 }),
-                #[cfg(feature = "mime")]
-                mimes: vec![],
             };
             ret.default_configure();
             ret
@@ -3525,12 +3525,16 @@ impl<H> Easy2<H> {
         Mime::new(self)
     }
 
-    pub(crate) fn mimepost(&mut self, mime_handle: *mut curl_sys::curl_mime) -> Result<(), Error> {
-        self.mimes.push(mime_handle);
+    pub(super) fn mimepost(&mut self, mime: MimeHandle) -> Result<(), Error> {
+        assert!(self.inner.mime.is_none());
+
+        let rc =
+            unsafe { curl_sys::curl_easy_setopt(self.raw(), curl_sys::CURLOPT_MIMEPOST, mime.0) };
+
+        if rc == curl_sys::CURLE_OK {
+            self.inner.mime = Some(mime);
+        }
 
-        let rc = unsafe {
-            curl_sys::curl_easy_setopt(self.raw(), curl_sys::CURLOPT_MIMEPOST, mime_handle)
-        };
         self.cvt(rc)
     }
 }
@@ -3548,11 +3552,6 @@ impl<H> Drop for Easy2<H> {
     fn drop(&mut self) {
         unsafe {
             curl_sys::curl_easy_cleanup(self.inner.handle);
-
-            #[cfg(feature = "mime")]
-            for &mime_handle in self.mimes.iter() {
-                curl_sys::curl_mime_free(mime_handle);
-            }
         }
     }
 }

src/easy/mime.rs

@@ -1,35 +1,49 @@
 use crate::easy::Easy2;
 use crate::error::Error;
 use curl_sys::{
-    curl_mime_addpart, curl_mime_data, curl_mime_filename, curl_mime_free, curl_mime_init,
-    curl_mime_name, curl_mime_type, curl_mimepart, CURLcode, CURLE_OK,
+    curl_mime, curl_mime_addpart, curl_mime_data, curl_mime_filename, curl_mime_free,
+    curl_mime_init, curl_mime_name, curl_mime_type, curl_mimepart, CURLcode, CURL, CURLE_OK,
 };
 use std::ffi::CString;
 use std::marker::PhantomData;
-use std::ptr::null_mut;
+
+#[derive(Debug)]
+pub(super) struct MimeHandle(pub *mut curl_mime);
+
+impl MimeHandle {
+    fn new(easy: *mut CURL) -> Self {
+        let handle = unsafe { curl_mime_init(easy) };
+        assert!(!handle.is_null());
+
+        Self(handle)
+    }
+}
+
+impl Drop for MimeHandle {
+    fn drop(&mut self) {
+        unsafe { curl_mime_free(self.0) }
+    }
+}
 
 #[derive(Debug)]
 pub struct Mime<'e, E> {
-    handle: *mut curl_sys::curl_mime,
+    pub(super) handle: MimeHandle,
     easy: &'e mut Easy2<E>,
 }
 
 impl<'a, T> Mime<'a, T> {
     /// Create a mime handle
-    pub(crate) fn new(easy: &'a mut Easy2<T>) -> Self {
-        let handle = unsafe { curl_mime_init(easy.raw()) };
-        assert!(!handle.is_null());
+    pub(super) fn new(easy: &'a mut Easy2<T>) -> Self {
+        let handle = MimeHandle::new(easy.raw());
 
         Self { handle, easy }
     }
 
     /// Finalize creation of a mime post.
-    pub fn post(mut self) -> Result<(), Error> {
-        // once giving the mime handle to `Easy2` it is now their responsibility to free the handle.
-        // so we need to make sure `Drop` below won't try to free it.
-        let mime_handle = self.handle;
-        self.handle = null_mut();
-        self.easy.mimepost(mime_handle)
+    pub fn post(self) -> Result<(), Error> {
+        // We give ownership on `MimeHandle` to `Easy2`. `Easy2` will keep record of this object
+        // until it is safe to free (drop) it.
+        self.easy.mimepost(self.handle)
     }
 
     /// Append a new empty part to a mime structure
@@ -38,15 +52,6 @@ impl<'a, T> Mime<'a, T> {
     }
 }
 
-impl<E> Drop for Mime<'_, E> {
-    fn drop(&mut self) {
-        // we only need to free mime handles which hadn't been given to the ownership of `Easy2`.
-        if !self.handle.is_null() {
-            unsafe { curl_mime_free(self.handle) }
-        }
-    }
-}
-
 #[derive(Debug)]
 pub struct MimePart<'a> {
     handle: *mut curl_mimepart,
@@ -56,7 +61,7 @@ pub struct MimePart<'a> {
 
 impl<'a> MimePart<'a> {
     fn new<E>(mime: &mut Mime<E>) -> Self {
-        let handle = unsafe { curl_mime_addpart(mime.handle) };
+        let handle = unsafe { curl_mime_addpart(mime.handle.0) };
         assert!(!handle.is_null());
 
         Self {
@@ -110,3 +115,33 @@ fn code_ok(code: CURLcode) -> Result<(), Error> {
         Err(Error::new(code))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::easy::Easy;
+
+    /// Trivial test which checks that objects can be used as planned.
+    #[test]
+    fn test_ownership() {
+        let mut easy = Easy::new();
+        let mut mime = easy.add_mime();
+
+        for i in 1..5 {
+            let name = format!("name{i}");
+            let data = format!("data{i}");
+            let fname = format!("fname{i}");
+
+            mime.add_part()
+                .set_data(name)
+                .unwrap()
+                .set_data(data)
+                .unwrap()
+                .set_filename(&fname)
+                .unwrap()
+                .set_content_type("plain/text")
+                .unwrap();
+        }
+
+        mime.post().unwrap();
+    }
+}