Add the new tasks with most of the job done

crates/milli/src/update/upgrade/mod.rs

@@ -0,0 +1,65 @@
+use crate::constants::{VERSION_MAJOR, VERSION_MINOR, VERSION_PATCH};
+use crate::progress::{Progress, VariableNameStep};
+use crate::{Index, Result, UserError};
+
+pub fn upgrade(index: &Index, base_version: (u32, u32, u32), progress: Progress) -> Result<()> {
+    let wtxn = index.env.write_txn()?;
+    let from = index.get_version(&wtxn)?;
+    let upgrade_functions =
+        [(v1_12_to_v1_13 as fn(&Index, Progress) -> Result<()>, "Upgrading from v1.12 to v1.13")];
+
+    let current_major: u32 = VERSION_MAJOR.parse().unwrap();
+    let current_minor: u32 = VERSION_MINOR.parse().unwrap();
+    let current_patch: u32 = VERSION_PATCH.parse().unwrap();
+
+    let start = match from {
+        // If there was no version it means we're coming from the base version specified by the index-scheduler
+        None if base_version.0 == 1 && base_version.1 == 12 => 0,
+        Some((1, 12, _)) => 0,
+
+        // --- Error handling
+        None => {
+            return Err(UserError::TooOldForUpgrade(
+                base_version.0,
+                base_version.1,
+                base_version.2,
+            )
+            .into());
+        }
+        Some((major, minor, patch)) if major == 0 || (major == 1 && minor < 12) => {
+            return Err(UserError::TooOldForUpgrade(major, minor, patch).into());
+        }
+        Some((major, minor, patch)) if major > current_major => {
+            return Err(UserError::CannotDowngrade(major, minor, patch).into());
+        }
+        Some((major, minor, patch)) if major == current_major && minor > current_minor => {
+            return Err(UserError::CannotDowngrade(major, minor, patch).into());
+        }
+        Some((major, minor, patch))
+            if major == current_major && minor == current_minor && patch > current_patch =>
+        {
+            return Err(UserError::CannotDowngrade(major, minor, patch).into());
+        }
+        Some((major, minor, patch)) => {
+            return Err(UserError::CannotUpgradeToUnknownVersion(major, minor, patch).into())
+        }
+    };
+
+    enum UpgradeVersion {}
+    let upgrade_path = &upgrade_functions[start..];
+
+    for (i, (upgrade_function, upgrade_msg)) in upgrade_path.iter().enumerate() {
+        progress.update_progress(VariableNameStep::<UpgradeVersion>::new(
+            upgrade_msg.to_string(),
+            i as u32,
+            upgrade_path.len() as u32,
+        ));
+        (upgrade_function)(index, progress.clone())?;
+    }
+
+    Ok(())
+}
+
+fn v1_12_to_v1_13(_index: &Index, _progress: Progress) -> Result<()> {
+    Ok(())
+}